如图所示的各种装置，在使用过程中都可视为杠杆，其中属于费力杠杆的是（ ）
A．
B．
C．
D．
根据省力杠杆、费力杠杆、等臂杠杆的定义可得：
选项A中的羊角锤，在使用的时候，动力臂大于阻力臂，是一个省力杠杆．
选项B中的镊子，在使用的时候，很显然动力臂小于阻力臂，是一个费力杠杆．
选项C中的瓶盖起子，在使用的时候，动力臂大于阻力臂，是一个省力杠杆．
选项D中天平，两边的力臂是相等的，所以动力臂等于阻力臂，因此是一个等臂杠杆．
可见，只有选项B属于费力杠杆，故选B．
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| In the doctor's waiting room, sick men and women were sitting on the chair. Bob, a school boy, was among them. They all looked very sad 1 Bob. He lost himself (沉浸于) in an 2 story in a magazine. Just then the doctor came in and said he was ready 3 the next person. Bob jumped up and 4 the doctor's room. "What's your trouble?" asked the doctor. Before Bob could say a word, the doctor made him 5 on the bed. "Now I'll listen to your heart beat." Bob tried to speak, but the doctor told him not to 6 anything. "I'll 7 your temperature." Bob tried 8 , but the doctor stopped him. After a moment, the doctor said, "Well, boy, don't 9 . There is 10 with you." "I know," said Bob. "I just come to fetch (取) some medicine for my father." | ||||
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