问题补充：
Which sentence is wrong?
A. We don’t think it necessary to give students tests every week.
B. We don’t have to worry about not having enough to eat now.
C. My father has left school for over twenty years.
D. He was heard singing a pop song when I passed his room.
答案：
A. We don’t think it necessary to give students tests every week.
B. We don’t have to worry about not having enough to eat now.
(答案→)C. My father has left school for over twenty years.
D. He was heard singing a pop song when I passed his room.
解析：答案C是错误的。因为leave不是延续性动词，在现在完成时态中不能与一段时间状语for over twenty years连用。而答案D是正确的。主动句可为：I heard him singing a pop song when I passed his room.
