下列各句中，表达得体的一句是（ ）

sked to explain ________ he does to makehis students so fascinated with his lessons, the teacher paused and thoughtdeeply

It is by no means clear ________ the president can do to end the strike.(2012·新课标全国)

The man is feeling much better now, so you ______ call a doctor.

The winner is a tall and thin boy with glasses!—Well，it________ be Mike. He doesn’t wear any glasses.