Choose the best answer

16. The little girl asked what _______ to her friend.
A. has happened B. happened
C. had happened D. would have been happened
17. John ______ a book when I saw him.
A. is reading B. read C. was reading D. reading
18. He said he _______ return later.
A. will B. would C. can D. would be
19. Jack ________ the door.
A. has just opened B. open
C. have opened D. opening
20. I have been waiting for you ______.
A. since early morning B. since 9 a. m
C. for two hours D. All are correct