It             be the vocabulary that caused you the problem in the exercise because you know a lot of words.

— Li Fan won the first prize in the national physics competition. — I ________ he was the winner.

There's enough time for you to go to the airport.You        hurry now.

 I’m feeling much better now, so you           call the doctor.

—Do you think the rain will affect the football match?—The players are used to such rainy weather, so it ______ make any difference to them.