Many people think that it is making such an experiment.
--Who______ it be that is knocking at the door? --It ______ be Father, but I’m not sure.
Judging from his______ expression, this problem must be quite ______.
Listen! Do you hear someone _____ for help?
--How is your cousin, Kate? --She broke her leg and ______ medical treatment since.
--Mom, may I play computer games just for a while? --______! Dad is writing his paper.