By the time you finish the book, you _______________ more about Chinese history.

A．will learn

B．have learnt

C．will have learnt

D．are learning

当气流速度较小时，空气深层过滤除菌起主要作用的是：（）。

A.惯性冲击滞留

B.拦截滞留

C.布朗扩散作用

D.重力沉降作用