问题
选择题
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
B
“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。