I won’t tell the student the answer to the math problem until he ______ on it for more than an hour.
He________have completed his work; otherwise, he wouldn't be enjoying himself by the seaside.
He paid for a seat, when he________have entered free.
________such heavy pollution already, it may now be too late to clean up the river.
John, look at the time,________you play the piano at such a late hour?
—Are you coming to Jeff's party? —I'm not sure. I________go to the concert instead.