21. I knew he failed in the entrance examination the moment I ______ him crying alone in his room.
A. at the sight of B. caught sight of
C. in sight of D. lost sight of
【解析】选B。我一看见他独自在房间里哭，就知道他没有通过入学考试。catch sight of看见；lose sight of看不见；A、C两项都是介词短语，不能作谓语动词。
22. He wrote her a long letter expressing the fact ______ she would not misunderstand him.
A. so as to B. in order to
C. so that D. for fear that
【解析】选C。句意为：他给她写一封长信来表达这个事实，目的是为了让她不误解他。so that在这里意为“为了”，引导目的状语从句。
