题干
When I tried to find _________that prevented so many people from taking part in the program, it seemed to me that there were two main reasons．
A：why it did
B：that it did
C：what it wa
D：why it wa
答案(点此获取答案解析）
C
＝a，
＝b，
＝4
，P为AD的中点，则
＝( )
