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In which situation is friction useful?

下列哪项是摩擦力的用处？

I。  Sharpening a knife 把刀磨利

II。 Producing heat in a machine 使机器产生热能

III。Holding an object 握住一个物体

IV。Waste of energy 浪费能源

A。I and III

     I 和 III

B。I and IV

     I 和 IV

C。II and III

     II 和 III

D。II and IV

     II 和 IV

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