They waited ( ）it was dark before leaving, as they didn't wan

They waited ( ）it was dark before leaving, as they didn't want anyone to see them leave.
A、since
B、so
C、unless
D、until
【正确答案】：D
【题目解析】：考核关联词含义，句子含义是离开之前他们一直等到天黑，因为他们不想让别人看到他们离开。