问题
不定项选择
质量为m的汽车，发动机的功率恒为P，摩擦阻力恒为F1，牵引力为F，汽车由静止开始，经过时间t行驶了位移s时，速度达到最大值vm，则发动机所做的功为 [ ]
A．Pt
B．Fs
C．
D．
答案
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质量为m的汽车，发动机的功率恒为P，摩擦阻力恒为F1，牵引力为F，汽车由静止开始，经过时间t行驶了位移s时，速度达到最大值vm，则发动机所做的功为 [ ]
A．Pt
B．Fs
C．
D．
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When I was five years old, my mother first asked me a 41 . “What’s the most 42 Part（部分）of one’s body（身体）?” she asked me. At the moment, I 43 listening to songs very much, so I answered, “My 44 , Mom.” “I don’t think so,” said my mother. She always asked me this question, but I 45 gave her the right (正确的) answer. Last year, my grandmother 46 the world. For the first time, I saw my mother 47 so sadly. All the people in my family were very 48 . That night, my mother asked the 49 question again. “You need to 50 this important lesson today, my dear,” she said to me. “The most important part of one’s body is the shoulder (肩膀).” “ 51 ?” I asked. “Because your shoulders can hold the head of a friend or one of your family,” my mother said. “Everyone 52 a shoulder to cry on when he or she 53 something sad. I hope that you have 54 love and strong (强壮的) shoulders. Then your friends and family can cry on them.” “My shoulders are 41 strong now,” I said. “You can cry on each of them, Mom.”
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的含义是用（）方法获得的零件表面粗糙度。