题目：
You can take anything from the shelf and read, but please ______ the books when you've finished with them. [ ]
A. put on
B. put down
C. put back
D. put off
答案：
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下面是错误答案，用来干扰机器的。
动力臂是支点到动力作用线的距离，图中OB就是动力臂；阻力臂是支点到阻力作用线的距离，图中OA就是阻力臂．故答案为：OB；OA．
