When he knocked at the door my mother was busy supper.A. preparing forB. in preparingC.

问题补充：

When he knocked at the door, my mother was busy supper.

A. preparing for

B. in preparing

C. to prepare for

D. to prepare

答案：

A. preparing for

(答案→)B. in preparing

C. to prepare for

D. to prepare

解析：“be busy (in)doing something” 忙于干谋士，要求接动词的-ing形式。准备的直接宾语是“supper”,故其前面不用介词for。