Susan decided not to work on the program at home because she didn’t want her parents to kn

问题补充：

Susan decided not to work on the program at home because she didn’t want her parents to know what she.

A. has done

B. had done

C. was doing

D. is doing

答案：

A. has done

B. had done

(答案→)C. was doing

D. is doing

解析：过去进行时可表示过去某段时间一直进行的动作。