—— Ann is in hospital.

____ Oh ,really? I ______ know. I ______ go and visit her.

A．didn’t, am going to

B．don’t , would

C．don’t, will

D．didn’t, will

竖直平面内固定一内壁光滑半径为r的圆形细弯管，如图所示．管内有一质量为m，直径很小的小球（可视为质点）做圆周运动，小球在最高点时，恰与管壁无相互作用力，则小球通过最低点时管壁对小球的作用力大小为（　　）

A．4mg

B．5mg

C．6mg

D．7mg