The sphere fighters carried William and his party in the sea. William suddenly felt a special smell ahead of him. He sniffed carefully, and immediately covered his nose and mouth, coughing and choking. "Oh my God," He said, not daring to believe the data the robots in the backpack had worked out, "excess levels of hydrogen sulfide?! Are we going to get out alive?"

"Master, we're almost there." Aria said. She could already see the outline of the sunken ship.

Soon they were near the ship. After searching around the hull, they found the only door. That meant they had to pass here when going both in and out.

In order to keep everyone safe, William ordered the little robot to paint the invisibility coating on the sphere fighters as well. "Now, dear sphere fighters, you must hide, same as us." As William said these words, those spherical shadows vanished.

William went first, and he opened the door with the sphere fighter. A strong stench rushed over the party at once. They were coughing violently, but managed to control themselves before entering the hull. "If we're too loud and there's danger inside, we'll be exposed!" Ranran said.

The sphere fighters entered the dark hull in turn.

Sphere fighters lit up the interior with dim lights. They almost screamed: the floor was a heap of ghastly bones, dyed red by blood. The walls were spattered with red spots of blood. Some of the bones were strange, because they looked like bones of birds. All this reminded them of what they had seen in Black Tower, and they shivered violently.

They found something useful. It was written in blood:

A certain poison causes disease by corroding the bones. Which of the two skeletons nearby may have been killed by this poison? After selecting, gently press on the head.

So the party watched with the help of the light. The skeleton on the left had relatively intact bones, while the one on the right had very little left. So the answer was obvious.

William wanted to go ahead and operate, but Natalie stopped him. "Master, it would be better if I tried." She took control of the sphere fighter's robotic arms and pressed down on the skeleton's head.

Suddenly a green light went on. Almost immediately, all the sphere fighters quickly faced that green light. The party's nerves were tense.

"SHH! Look, there's a note." Photon glimpsed a corner of paper under the skeleton. Natalie's sphere fighter quickly picked it up while the others read in whispers,

It's normal for you to be nervous. But for now you are safe. However, please notice that after you complete every small task, the green light will go on. Don't make any movement for the next 20 seconds when the green light is on! The standard for movement to occur is that the speed is greater than or equal to 1 km/h. This time, time counting will start from the moment you put this note down -- otherwise the light will turn yellow. If no movements occur within 10 seconds after the green or yellow light is on, the light will go off; however, if you move when the yellow light is on, you will have a red light! Never trigger the red light, or you will...

The following writing was so faint that it was impossible to read it. But their intuitions told them that was no good.

They figured out the solution: everyone else should stare at Natalie's sphere fighter to see when she'd put the note down completely, and then wait for 20 seconds without moving.

Natalie put down the note, and then all the sphere fighters froze. Everyone stared at the light, praying that the time would pass quickly.

The light finally went out. They continued to search for useful objects. The lights of the sphere fighters hit a jigsaw puzzle. Photon controlled the sphere fighter and picked it up. Then he saw "1/3" written on the back. "That means there are still two pieces of the jigsaw that have not been found." He said to the others.

Ranran found another note.

The stopwatch in the picture has an accuracy of 1 second. Please estimate a possible value of the angular speed of the windmill.

William thought carefully. "Wait a minute." He said, "They didn't tell you which direction the windmill span in, did they?"

"No." Said Rebecca.

"That's good. The angular speed is a vector, which means that if one direction is positive, the other is negative. Let's just assume that clockwise is positive. As you can see, because the windmill has three blades, it re-shows its shape every 120 degrees of rotation. When you see the same shape as last time, it may have rotated by an integral multiple of 120 degrees. The phase difference of the two shapes in the pictures is 60 degrees, so it may rotate by an odd multiple of 60 degrees. Let's write Pi /12 radians per second."

Their answer was correct. The green light came on, and they knew what to do. When the lights went off, they picked up a piece of jigsaw marked "2/3".

"Only one piece of the jigsaw left, come on everyone!" Cheered up Ranran.

The sphere fighters moved slowly in the dark hull. They grew used to marching among skeletons and blood. The water seemed to freeze, and the pungent smell grew stronger. William was even more nervous because he didn't know what would happen when all the pieces of the jigsaw were collected. As a result, his sphere fighter was always ready to fire.

At last they saw the note saying the third question,

For any positive integer, if it is odd, times 3 and then plus 1; if it's even, divide by 2. If you iterate, you will end up with the same and smallest number whatever the original number or however many times you iterate. When you get that number, stop iterating. What is that number?

"Oh? Interesting." Thought William, "Let's take a particular example."

He used 15 as an example. "The first time we get 46, the second time we get 23, the third time we get 70... Oh? There's an infinite loop including 4, 2 and 1? So let me verify it with even numbers."

He chose 42. The same infinite loop appeared once more. "Ah, it's wonderful. So now I need to figure out which of these three numbers it is. Given the words 'any positive integer' and 'smallest number', I think this number should be 1. Because if you pick any other number, they all can turn into 1, and it really is the smallest number in the loop."

The green light came on again. 20 seconds later, they found the third piece of the jigsaw.

"Where are we going to put these pieces of jigsaw?" Aria asked.

"Look! There." Natalie spotted a gaping gap. Its shape fit perfectly the outline of each piece of the jigsaw.

So William carefully operated the sphere fighter, putting the pieces into place. The jigsaw glowed faintly, then the gap opened to reveal a notebook.

Ranran's sphere fighter quickly took it out. "There's writing in it!" She said.

They all went and read,

Help!

Our species is going extinct, and it's due to killings of unidentified creature! We desperately need to be rescued, even if you only keep our genes!

Please complete this sudoku sheet and you will know all about us!

1 4 3 2 2 1

"Oh. It turns out to be a 4 by 4 sheet. Then it will be easier." William thought for a moment, then filled out the form:

1 2 3 4 4 3 1 2 2 1 4 3 3 4 2 1

On the back of the page was information about the person who had written the request:

Charlotte, female red seagull, gentle. Now located in Hobbs Cave, having a population of just 5,000. Support is urgently needed.

"Really?" William took the notebook and exclaimed, "Only 5,000? Wouldn't they be extinct in a flash? This is terrible..."

"Yes, it's terrible!" Natalie said, "We must find Charlotte as soon as possible."

"I suggest we go there as soon as we get out!" Photon said. Rebecca and Aria agreed.

Rebecca saw the last note.

Congratulations! You have finished all the tasks successfully. Now, it's time to test your ability to move slowly. The green light will remain on until you are all out of this sunken ship. However, you can move at no more than 5 km/h. You don't have to go out so fast, but don't speed! Or you will...

"Damn! Why are the words still illegible!" Muttered William angrily.

The sphere fighters quickly lined up and slowly flew in turn toward the entrance. The party all controlled the speed with great nervousness. Their blood was almost frozen. As before, time seemed to be frozen. Every second seemed like an eternity. William was at last place because he wanted to keep the people in front safe all the time, and because it made it easier for him to monitor the light.

Ranran, who was in front of William, was already out, and he was almost at the exit. The others all looked at him with bated breath. "Hold on, Master!" Natalie said to herself nervously.

"Don't worry, you surely can do it." Whispered Photon.

William nodded, closing his eyes nervously. "I dare not look back," He said to himself. His hand held the accelerator lever stiffly, his heart racing, his breathing quickening. Ranran could clearly see that his neck already exposed blue veins, and his forehead was also soaked with sweat.

William was able to slowly move the sphere fighter out of the sunken ship. "Run! Don't hang around here another second!" He shouted. The exhaust ports of the sphere fighters glowed with bright light, and then, in the blink of an eye, they were hurtling toward the surface of the sea.

Unfortunately, they did not hear an angry voice from inside the ship,

"Why did they escape again! Damn it, I'll kill them!"