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?
问题描述：
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
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.