21．On Sept. 29, 2008，Shen Zhou VII returned to _______ earth from _______ space successfully.
A. the; / B. /; the C. /; / D. the; the
22. The number of people who _________ cars _________ increasing in China.
A. owns, are B. owns, is C. own, is D. own， are
23. Is this the reason _________ at the meeting for his carelessness in his work？
A. he explained B. what he explained
C. how he explained D. why he explained
