21．It is believed that traveling to Yangzhou in____March is____enjoyable experience.
A．/; the B．the; an C．/; an D．the; the
22．Although it was really a difficult task, he decided to try his best to____ success.
A．challenge B．host C．approve D．achieve
23．I found the homework was not so heavy as ___I ___ to get in my old school.
A．that; was used B．what; used C．which; used D．that; used
24．Have you read the bookI lent to you?
A．that B．whom C．when D．whose