Nervously, the party climbed the spiral staircase to the second floor. They had barely left the stairs when they heard the faint footsteps of the robots. Sure enough, not far ahead, a robot approached them. But unlike the previous one, it was getting slower and slower.

"Pretty good, we had time to react." William thought and fired. The robot fell to the ground.

William had no trouble getting the green key. The compass read,

"Yes, the robot is slowing down, consistent with the monotony of this function." He thought.

Compared with the spacious living room and smaller bedrooms on the first floor, the structure on the second floor was more complicated: the corridors were narrow, while the bedrooms and storage rooms were large. Based on his experience, William suspected that the remaining three robots were more likely to patrol spacious bedrooms or storage rooms than narrow corridors. "It won't make it easy for them to extend their patrols," He convinced himself.

Thinking, they went into another bedroom. The party was frightened out of their wits, nearly screaming -- there was a skeleton on the bed, sleeping peacefully, as if it did not know that it would never wake up again. The blood dyed all its bones and gave off a faint glow of heat. From its posture, and from the temperature of its blood, they thought the poor guy died just now. But they wondered why all that was left was bones.

Aria said nervously, "Looks like this was a death caused by an unfamiliar weapon."

Then the footsteps of the robot came from the corner. Its blood-red eyes were searching everywhere. Though it couldn't spot them at all -- it was impossible unless it bumped into them -- it somehow sensed that something was moving nearby.

As the robot patrolled, it had no idea that numerous muzzles had been pointed at its terrifying body. It just continued to follow its own trajectory. But the party found the robot's trajectory strange: in addition to approaching them, it was moving sideways, and at regular intervals it repeated its movements.

"What is it trying to do?" William stared at it as he nervously prepared to pull the triggers.

That was close enough. A torrent of bullets poured out, hurtling into the robot's terrifying body. Its electrical system was burned, so it could no longer work and fell on the floor.

"Let's see what the compass shows this time." As William went to fetch the key, he took note of the compass shell. After getting the blue key, the words on the compass became,

"Oh, no wonder it goes sideways from time to time." William saw the light. Recalling carefully, he was convinced that this function could describe the trajectory of the robot.

They walked quickly out of the bedroom with the dead skeleton and into the corridor again. There were two robots left who were on patrol.

By the light of the compass, the party made slow progress in the dark of terror and silence. All nerves stretched like tensed bows, not daring to let down their guard. Frightened, Aria looked around for the robots. Photon and Rebecca were ready to fire at any time. Natalie aimed with a laser gun on her hood at the terrifying darkness. They stayed close to each other, not daring to leave.

The party moved at a snail's pace into the storage room. William and Ranran both put out first one foot, then the other. There was not so much blood as in other places, but they were still repressed. A tall black locker towered by the wall and stretched its arms to the other side of the room. There were also many different sizes of lockers beside the walls.

The party heard a strange noise coming from a locker in a corner. Immediately, everyone raised their laser guns and aimed them at the locker. "I bet there are robots in there." William thought as he held the guns nervously.

Yes, there was indeed a robot hidden in the locker, but it took a long time for it to come out. No one knew why. Maybe it felt them? But they were already invisible, so only the sense of touch still worked. But the robot didn't touch them, so how did it feel them?

The robot stepped out of the locker and approached William quickly, then slowed down, then picked up speed again. Again, it was going to follow a different trajectory. "I'd like to see how many tricks you have. But you'll soon --" His words were barely finished when gunshots rang out. Then came the sound of the robot falling to the ground.

William went over to get the key. He found the cyan key on the robot's remain, and the compass showed this.

"Oh?" William thought for a moment, "It's a new function. Yes, that's how it moved."

Now, only one robot remained to be killed. The faster they killed it, the sooner they could get out of this terrible house.

So they went into the corridor once more and went to the final bedroom, which was the largest in the house. They could already see the back door not far ahead, and could see through the window that there was a slide connected to the back door.

Two minutes elapsed from the moment William, at the front, entered the bedroom to the moment Aria, at the back, entered. A terrible silence surrounded them. If they weren't wearing the "completely disappear" clothes, their breath would be deafening, which would no doubt startle the robot. Ranran was glad they had met the white spider robot.

The compass glowed bright red. On the outside of its shell appeared the following words.

Be careful. This robot's trajectory could be complicated. Always be ready to fire.

Seeing this, William immediately touched the others and told them to keep an eye every second.

They waited a full minute, but they did not see the robot.

"Watch carefully and don't let down your guard!" William reminded the others.

The robot finally appeared. Its blood-red eyes glowed wickedly, and its pale bones moved mechanically and feebly. If they had come any closer, they would have been terrified, for the robot was hideous. Its trajectory was indeed very complicated -- it moved in a straight line, but it also moved in a curve; it darted toward the party, then slowed down again. In short, they could find almost all the trajectories they had just seen.

"We have to find a position to attack. Let me see what law there is." William pointed his guns at the robot.

The robot kept walking until it was about a meter in front of them. Then it turned around, went back, and repeated the actions it did just now. William could not take his eyes off it.

"Oh! I think I've found the law." His brain was racing, "It changes track about every five seconds. Each loop consists of six small parts which are the very little parts that we just saw."

He had to decide when it was best to fire. "I thought I'd wait until it went back, because even if we didn't hit it, it would take time for it to turn around. We can use that time to attack again, or we can hide. Twice in a cycle it moves in a straight line. There's no denying that this timing is perfect."

He told the others of his plans. They all agreed. Now, all they had to do is to wait for the robot to approach them and turn around, and then fire immediately.

The robot approached the party again. They didn't care about its complicated trajectory. They just wanted to kill it quickly. It looked like Ranran's heart was about to fly out of her chest the next moment, while Photon and Natalie froze like stones, not daring to move. Rebecca and Aria were glued to each other. The most nervous, certainly, was William, who was waiting for the perfect moment.

It finally turned around. Then, in a flash, a hail of bullets with fear, anger and so on pierced into the body of the robot. In an instant, its electrical system was destroyed. As a result, it twitched wildly and then fell heavily to the floor a few seconds later.

All the robots had been killed. William got the final and purple key and the answer from the compass:

"Oh! No wonder its trajectory looked so complicated; it turns out to be periodic piecewise function!" William thought.

At once they went to the back door. William saw that there were seven colored keyholes in the door. They needed to insert the corresponding keys.

William took out all the keys, lit them with the compass, and quickly inserted seven keys. Then the lock slid to one side, revealing the writing hidden behind it:

Hang in there for one more second, my friend. Answer this correctly, and you can go out.

What kind of paint is needed to make the house look eerie?

A that absorbs all electromagnetic waves B that absorbs long waves C that absorbs short waves D that does not absorb all electromagnetic waves

"Oh, that's nothing." Said William, "We can see objects because they emit or reflect light which goes into our eyes and the optic nerve sends signals to the brain. Black doesn't reflect any electromagnetic waves. The answer is choice A."

He clicked "A" and the back door opened slowly. The party saw a long slide ahead. It was big enough in diameter to hold Aria.

The party immediately lined up and taxied downward. They had been long waiting for going out.