－ Whose is this blue bike? －It ________ be Mary’s. Her bike is black and white. 

---Do you still remember when we went to Chong qing?----I can’t remember now but_____ sometime last summer?

-- _____ I ask you a question, Mr. Wang?      -- Yes, you _______.

---Who is the tallest boy?        ---It ______  be John. John is of medium height.

—Why didn’t Sam attend the party yesterday evening?—He ___ not have wanted to see me.