题目内容
（请给出正确答案）
[主观题]
If a needle is thrown ______ on a sheet of paper ruled with lines, how often can it be exp
ected to fall on a line？
A．on purpose
B．with accuracy
C．at random
D．according to one's own will
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起重机滑轮组中动滑轮的作用是（）。
A.改变钢丝绳受力方向
B.省力
C.平衡
