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.