几道英语选择题．．精通的请帮我分析下

1:He worked hard to ___ some money for his retirement.
A.set aside B.set out C.set off D.set about
选A。
set aside留出；set out出发；set off开始；set about着手。

2:Out of ___ for the homeless children , he gave them shelter for the night.
A.pity B.shame C.sympathy D.mercy
选C。怀着恻隐、同情之心。pity 遗憾；shame 羞愧；sympathy 同情；mercy 善良。

3:After the explosion , the policeman did a lot of work to find out some clues from the___.
A.lookers-on B.viewers C.people D.witness
选D。witness 目击者。lookers-on 旁观者。
这两个词从汉语里就可以比较它们意思的不同啦。
另外补充个谚语，Lookers-on see more than players. 当局者迷，旁观者清。

4:Only then did he ___ the effects of his action.
A.aware B.aware of C.become D.become aware of
选D
1)在句首的only如果后面跟了副词、介词短语或从句等状语，则主句用倒装结构。如only后跟的是主语，就不用倒装结构了。
2)aware of与aware前都要跟be，而此处的句子还原回去主语后跟did而不是was，故不选A/B。
3)C在此处使句意不通顺，故选D。

5:He was about to go out for a walk ___ someone knocked at the door.
A.then B.when C