3.They waited _____it was dark before leaving,____they didn’t want anyone to see them leave.

问题描述:

3.They waited _____it was dark before leaving,____they didn’t want anyone to see them leave.
A.since;because B.until;as C.unless;for D.so;because

他们知道天黑才离开,因为不想被任何人看到他们离开.单看第二句,三个单词均表示因为,都可以.根据句意是直到天黑,unless是除非的意思,不合题意.所以选B