Cancer has become the top killer ________ I think threatens the health of millions of people.

A number of birdwatchers ______in Zhalong Nature Reserve. The number of them____500.

I wonder if I could pay the bill by check.— ______. The card machine has just broken down.

Why don’t you go to visit the Great Wall with us—Because I _______ there many times.

What do you think of Peter—He ________ be really difficult to get along with even though he’s a nice person in general.