The teacher always tells her students that only if __________ hard _________ achieve success.

A．do they work, will they

B．do they work, they will

C．they work, will they

D．they work, they will

一木块重10牛，用3牛的水平拉力使其在水平桌面上做匀速直线运动，若改用5牛的水平拉力使其移动一段距离，则此时木块受到的摩擦力f和合力F的大小为（　　）

A．f=3牛  F=0牛

B．f=9牛  F=0牛

C．f=3牛  F=2牛

D．f=5牛  F=2牛