这道题一直想不通~!很纠结.急等解答 谢谢大家了~!

问题描述:

这道题一直想不通~!很纠结.急等解答 谢谢大家了~!
_____there are a large number of chemical factories, the air is likely to be polluted
A that B where C once D as
答案是B
为什么不选C“一旦…”或者D“因为…”

背了得了.
如果你知道这句话where there is a will,there is a way.凭语感就能选对