It is _____ he often fails in exams ________ makes his parents worried about him.
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.
Mr. Bush is on time for everything. How________it be that he was late for the opening ceremony?