题目内容：

Every object has weight．But the center of its weight is not always in the middle of the object．If the top of ail object is heavier than its bottom (底部)，it will fall over easily．We say that its center of gravity (重力) 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 (铁) 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．1ighter than                     B．heavier than

C．no heavier than                  D．as heavy as

2.Please point out which box will not fall．

 

3.The reason why the Leaning Tower of Pisa does not fall is that         ．

A．the top of the tower is heavier than its bottom

B．the bottom of the tower is not heavier than its top

C．its center of gravity is still above the line of its bottom

D．its center of gravity is not above the line of its bottom