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）

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.