—Who______ it be that is knocking at the door?   —It ______ be Father, but I’m not sure. 

You ________ pay too much attention to your reading skill, as it is so important.

 Grace doesn't want to move to New York because she thinks if she ________ there，she wouldn't be able to see her parents very often.(2012·安徽)

(2013·北京，34)If we________a table earlier, we couldn't be standing here in a queue.

—Why aren’t they here yet?—They           the bus.