题目

Jack was a heavy smoker. His mother didn't like him smoking，and 1.    he couldn't give it up. At about 9 o'clock in one evening, Jack was seen  2.   smoking in the living-room again. “Don't smoke in the living-room,  3.  Jack!”said his mother in an unhappy voice. Heared this，Jack went  4.   back his own room and shut the door. While he was sitting in bed,  5.   he couldn't help smoke again. About an hour later，Jack's mother   6.   smell heavy smoke. She got up and opened Jack's bedroom. Good heavens! 7.   Jack's quilt was burning while he was sleep！ She q如图所示，斜面长 L，高 h，现在用力F沿斜面把质量为m的物体从斜面的底端匀速拉到顶端，已知物体与斜面的摩擦力为f，则计算斜面机械效率的表达式可写为（ ）A．η=B．η=C．η=D．η=