(2013·长沙二模)—The research on the new flu virus vaccine is challenging  and demanding.Who do you think can do the job?—________ my students have a try?

— Must I finish the exercise at school? — No, you don’t have to. You          do it at home.

—Whose shirt is this?—It ______ be Li Lei’s. It is too big for him.

— Who’s that man in our classroom? It ________ be Mr. Brown. He is much taller.— You mean the man in white? He is an exchange student from England.

---Look! That’s Amy.   ---It _______ be her. She’s gone to Xi’an.