They were surprised that a four-year-old boy should work out such a difficult problem ____ they themselves couldn’t.
问题描述：
They were surprised that a four-year-old boy should work out such a difficult problem ____ they themselves couldn’t. A.once B.then C.while D.if
为什么选这个?
答
C.while这个句子的翻译：他们对于一个四岁的男孩能够做出这么难的题而感到惊讶,（ 当 ）他们自己不能做到的时候.通过理解句意,A一次,B然后 都不可能,排除…………由于句子是过去式,D如果也不对………故选C,当…的时候
选C因为:A一旦 B表并连,因果D表条件. 我们惊讶于四岁小男孩能做出这样难得题,然而他们本不应该会做的.while在此处是然而的意思,表转折,此处就算有BUT选项也不能选,因为两个独立成句之间的BUT连接必须有逗号连接.