Jack, look! Everybody is doing homework quietly. ________ you make a noise in class?

It doesn’t often snow in my hometown in winter, but it ___be rather cold sometimes. 

— It seems that Mr. Wu is standing there.— It ______ be him because he called me from Canada just now.

You _____ hurt your stomach if drinking beer while having sea food.

He ______ sleep, although he tried to, when he got on such a hunt for an idea until he had caught it.