谁能帮我解析一下这些题啊?1)It must have rained last night,_____the groun

谁能帮我解析一下这些题啊?
1)It must have rained last night,_____the ground is wet.
A.because B.as C.since D.for
正确答案是D,为什么不选A?
(2)____I haven't seen the film,Iknow very little aboutit.
A.After B.As C.Although D.Before
正确答案是B,为什么不选C
(3)The roof fell ____he had time to dash into the room to save his baby.
A.after B.as C.before D.until
正确答案是C,为什么不选B
(4)I suppose Bill is still studying in state university,_____?
A.do I B.don't I C.isn't he D.doesn't
正确答案是C,
(5)_____him,I was indeed very fortunate.
A.Comparing to B.To compare to
C.Compared with D.To compare with
正确答案是C,为什么不选D
(6)When I handed the report to John,he said that George was the person_____.
A.to send B.for sending it
C.to send it to D.for sending it to
(7)Many a man _____come to help us.
A.have B.has C.is D.are
正确答案是B