（ ）21. It's not_______good idea to go to school without _____breakfast.
A.a: 不填 B. the;a C.the;不填 D.a;the
（ ）22. With time____by, he came to understand how to behave,
A. going B. goes C. went D. to go
（ ）23.I guess, Peter will be back_____ a couple of weeks.
A. on B. in C. for D. by
