He was driving home on the highway last night when he was stopped by a policeman，         .

A．who was charged by speeding

B．who accused him of speeding

C．which was warned of speeding

D．whom reminded him to speed

图是一种电梯的结构示意图。A为电梯厢，B为滑轮，绕过滑轮的钢丝绳一端固定在楼房顶层，另一端通过电动机C提供拉力，D为配重。在电动机拉力作用下电梯厢能在电梯井中沿竖直通道上下运行。某次运行中，电梯厢以0.5m/s的速度匀速上升6m，将总质量为300kg的5个人由一层运送到三层，电动机拉力F为8000N。（不计钢丝绳、滑轮重及摩擦，g取10N/kg）

（1）在此过程中，拉力F做功的功率是多少？

（2）在此过程中，电梯的机械效率是多少？

（3）请你分析，配重的质量最好是多大？