— I'm afraid I have to leave a little earlier. My mother is ill in bed. 

— ____ I hope she will be all right soon. [ ]

A. It's nothing.

B. Nothing.

C. I'm sorry to hear that.

D. So she is.

下列说法正确的是（）

A.某接触面间产生弹力，则一定有摩擦力

B.某接触面间产生摩擦力，则一定有弹力

C.某接触面间产生的摩擦力方向与弹力方向一定垂直

D.猴子匀速往上爬杆，杆对猴子的摩擦力方向向下