下列说法正确的是（ ）
A．功率表示物体做功的快慢，所以做功时间越短功率越大
B．在相同的时间内做功越多，功率就越大
C．功率大的机械比功率小的机械做功一定多
D．有力作用在运动的物体上，力一定对物体做功
A、功率表示物体做功的快慢，在做相同的功时，做功时间越短功率越大，故A错误；
B、根据公式P=
可知，在相同的时间内做功越多，功率就越大，正确；W t
C、功率是表示做功快慢的物理量，不但和做功的多少有关，还与做功的时间有关，功率大的机械做功不一定多，故C错误；
D、做功的两个必要因素：一是有作用在物体上的力，二是物体在力的方向上移动一段距离，二者缺一不可，故D错误．
故选B
Eric was standing in front of a large, gray house in the countryside. He had just been dropped off by his mum, who left after telling him that he was going to the next twelve days there with no TV, no mobile phone and worst of all—no . Just then, a man appeared* at the gate of the house. When he saw Eric, he went up to him immediately*. "I'm you are Eric. Welcome to Camp Reboot," said the man proudly. "I'm Tony, the camp leader*, but you will me 'Sir'. Now you tell me you're here." Eric quickly replied, "My mum made me here!" When Tony looked angrily him and did not say , Eric realized his mistake and added, "Sir!" "And do you know why your mother sent here? Asked Tony. "She thinks I spend too much time online—that I can't live without the Internet... Sir," answered Eric. "It's not just her opinion. It's the ! Look at you—tired, unhealthy and certainly not getting sleep. You're here because you think your world on the Internet is much than the real world. But you're wrong! The real world has much more to offer. At Camp Reboot, you're going to go hiking, boating, rock-climbing and stay outdoors most of the ," said Tony. Eric's heart broke. "But it's not going be all fun and games." Tony continued. " We have rules here. you break them, you'll never be sent home ! Instead, you'll have to sweep the floor, wash the dishes or do an extra hour of exercise!" "Oh, my God!" cried Eric. "What have you got me into, Mum?"
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