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。