如图所示，动滑轮重0.5N，物体重3.5N，当OA=2OB时，为使轻质杠杆保持水平平衡、在A端所加的力应为（不计摩擦）

题干

如图所示，动滑轮重0.5N，物体重3.5N，当OA=2OB时，为使轻质杠杆保持水平平衡、在A端所加的力应为（不计摩擦）（）

A：0.5 N 　

B：1N

C：2N　

D：4N

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B

The job of the speaker	A____
The first thing that you have to do	Write a ____about yourself
The third thing to do	Read the paper to the____
Things you can't do	Don't ____that will make everyone laugh．
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