21. It was when we were returning home __________ I realized what a good feeling it was to have helped someone in trouble.
根据分析，此句是”It was…”开头，且去掉“It was “之后，句子结构仍然是一个完整意义的句子，故判断此句是一个强调句，强调“when we were returning home”。
【解析思路】强调句型：It is(was)+被强调部分+that(who)…句义：正是当我回家的时候，我才意识到帮助别人于危难之时感觉是多么令人愉快。故选B。
。对几种固定句型、从句的先行词的辨析。