1.
______ I admit there are differences between us,I don’t think they can separate us.
A.When B.If C.While D.As
2.
The meeting will be over _______4 o'clock in the afternoon.
A.until B.since C.in D.by
3.
Never hesitate to contact us— ____ you have any items that are puzzling you.
A.because B.unless C.while D.when
请帮忙分析说明一下选这些答案的理由和原因,