21．Language is a system of meaningful sounds ______ with a set of rules.
A. organizing B. organized
C. being organized D. having been organized
22．It was the Civil War, however, ______ made card photographs an American Practice.
A. when B. that
C. which D. why
23．Michael’s mother read him lost of stories about flying when he ______.
A. was growing up B. grew up
C. had grown up D. would grow up
