选择括号内恰当的单词或词组填入空白处，使句意完整、正确。（5分）
【小题1】Are you going to               （join / take part in）the school rowing club?
【小题2】I think the computer is               （used / useful）.
【小题3】I spent two hours                （sweep / sweeping）the house yesterday.
【小题4】The house is so expensive that I can’t               （share / afford）it.
【小题5】Even scientists aren’t sure               （if / whether）or not there are UFOs.

I don’t know     he will come tomorrow,     he comes, I will tell you.

I don’t know _____ or not he will come . _______ he comes , I will call you .

________ you can’t decide ________ to go or not, just stay at home and sleep all day.

     A. Whether, if                         B. If, whether                    C. If, weather

—Mary，I don’t know ____ you can come tomorrow morning.

—No problem! I’ll be here _____I’m free then.

A. if；if???????? ?? B. if; whether???? ?? C. whether；till? ? D. whether；until