从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
例：We______last night, but we went to the concert instead.
A. must have st udied B. might study
C. should have studied D. would study
答案是C。
21. ---Look! The telephone is broken. Someone damaged it ________ purpose.
---That may be right. But perhaps it was broken _________ accident.
A. on; by B. by; by C. on; on D. by; on
