when:you should be careful when _____ the street in big cities.

题目

when:you should be careful when _____ the street in big cities.
选项是:a.to cross b.cross c.crossing d.crossed
这个题目为什么选c呢?

答案

C
这句话其实相当于在the street之前省略了“you are".
把you are加进句子中,你就发现只能选C.