Two weeks ago, I bought a bicycle. If you live in town, it is often faster than a car and you           worry about parking. 

Sometimes smiles ______ be false, hiding other feelings like anger, fear or worry.

(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?

I _____thank you too much for all your help to my son while we were away from home.

-----Who ______ it be that hasn’t handed it in yesterday? ------It _______ be Bob. He handed it in yesterday.