---The light in his room is still on. Do you know _______?

---In order to get ready for the coming exam.

　 A. if he works hard　　　　　　　 B. why he stays up so late

　 C. why is he so busy　　　　　　 D. when he will stop working

If anybody calls, tell them I’m out and ask them to ______ their name and

addresses.

　　　 A. pass　　　　　　　　 B. write　　　　　　　　 C. take　　　　　　　　 D. leave

Every object has weight. But the center of its weight is not always in the middle of the object. If the top of an object is heavier than its bottom, it will fall over easily. We say that its center of gravity is high. But if the bottom is heavier, it won’t fall easily. Then we say that its center of gravity is low.

Look at the first picture. This empty box must fall over, because its center of gravity (C) is not above the line of its bottom (AB).

Now look at the second picture. There is some heavy iron in the bottom of this box. It will not fall, because its center of gravity is above its bottom.

1.If the bottom of an object is ______ its top, it will fall over easily.

A. lighter than　　 B. heavier than　　 C. no heavier than　　  D. as heavy as

2.Please point out which box will not fall.

3.The main idea of the passage is _________.

A. why things fall over　　　　         B. a low center of gravity

C. a high center of gravity　　             D. that everything has weight