如图所示是一种小型千斤顶，当手往下压动摇臂时，能把顶着的重物微微抬高一段很小的距离。把摇臂向上复位时，抬起的重物可以保持不动，再按下摇臂时，重物又被微微抬起。上下往复操作摇臂，重物便渐渐举高。用这种千斤顶把某汽车的一个后轮顶起时，施加在摇臂上向下的压力为100N，每压一次，手向下移动的距离是20cm，则每压一次手对千斤顶做功____ 焦，压120次后，后轮被抬高15cm，若汽车质量4×10³kg，千斤顶支持其重量的30％，不考虑手使摇臂向上复位所作的功，该千斤顶的机械效率是____ 。

根据短文理解, 从A、B、C三个选项中选择最佳答案填空。

    A thousand years ago, Hong Kong was covered by a  1 forest. As more and more people came to live in Hong Kong, these trees were 2 and burnt. Now there are only a few forests left and we call these 3, a small area covered with trees.

    Elephants, tigers and many 4 animals were living in the thick forest. Early farmers needed fire to keep themselves 5 in winter, to cook their food and to keep away the dangerous animals. They 6 rice and kept pigs and chickens in the valleys. They cut down the trees and burnt them. Elephants quickly disappeared 7 there was not enough food for them. So did most of the wolves and tigers. Monkeys and many other animals soon 8 in the same way.

    There are about 36 different animals living in Hong Kong now. One of the most interesting of Hong Kong's 9 is the barking deer. In Hong Kong the barking deer has only a real enemy human beings. There are now not 10 barking deer left. So it is important for people to protect wild animals.