题目内容：

请根据下面的实验过程给图排序，将其序号填入第66 ～69空格中，并完成第70小题。Do the experiment as follows ：

1.（  ）Put a short (0. 5 meter) board on two books. The books should be under the center of the board.

2.（  ）Ask your partner to stand on one end. With your hands ， push down on the other end. Can you lift your partner?

3.（  ） Put a longer (1 meter) board on the books. The end should be about 30cm from the books. Have your partner stand on the end that's near the books. Can you lift your partner?

4.（  ）Now do the same with the longest (2 meters) board. The end should be half a meter from the books. What will happen? Yes ！ You can lift your partner easily ！

5.What experiment is it?

A. A chemistry experiment.        B. A physics experiment. 

C. A math experiment.            D. A medical experiment.