21. My interest in stamp collecting______ from my schooldays.
A. is dated B. dates
C. had dated D. can be dated
【解析】选B。date from“追溯到……”不能用于被动语态，常用一般现在时态。
22. When I was young, I dreamed______ a teacher.
A. becoming B. of becoming
C. to becoming D. about becoming
【解析】选B。dream of doing sth. “梦想做某事”。
23. Some scientists are working hard at how tide(潮汐)can be________ to produce electricity.
A. harness B. harnessful
C. harnessable D. harnessed
【解析】选D。can be harnessed构成被动语态，故选D。
