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。 
Are you going to____ （join / take part in）the school rowing club?
I think the computer is ____（used / useful）.
I spent two hours____ （sweep / sweeping）the house yesterday.
The house is so expensive that I can't____ （share / afford）it.
Even scientists aren't sure ____（if / whether）or not there are UFOs.