—Please read every sentence carefully.      you are,      mistakes you’ll make.

—Thank you for your advice.

A．The more carefully; the fewer

B．The more careful; the less

C．The more carefully; the less

D．The more careful; the fewer

如图所示，建筑工人用滑轮组把重720N的钢材运送到离地5m高的工作台上，动滑轮重80N（绳重和摩擦不计）．在拉力F的作用下，钢材以0.1m/s的速度匀速上升．求：

（1）拉力F的大小？

（2）拉力F的功率？

（3）滑轮组的机械效率？