1.When I try to find out ______that prevents so many people from taking part in the program,it seems to me that there are two main causes.
A.what it is B.what it does C.why it does D.why it is
2.I'd appreciate_______you could give me some advice.
A.it B.that C.it if D.when
3.I hate it ________people speak with their mouths full.
A.if B.when C.because D.for
4.It was twelve o'clock _______they finished the work.
A.when B.at which C.that D.since
A,B,A,尤其是第二题it是不是做形式宾语,如果选that,那句子要怎么变化呢?第三题为什么不选because和for（for也不是有因为的意思吗?）第四题为什么不选at which?