What do you do when it ?- I often stay at home and read books.A. rainingB. rainC. rains

问题补充：

What do you do when it ?

--- I often stay at home and read books.

A. raining

B. rain

C. rains

D. is rains

答案：

A. raining

B. rain

(答案→)C. rains

D. is rains

解析：考察时态。when引导时间状语从句。从句描述的是反复性的动作，故用一般现在时态。主语是it，单数第三人称，故谓语动词要用单数形式。选C。