--Who is the man over there? Is it Mr. Li ?--No, it _______ be him. Mr. Li is much taller.
I was about to enter the office ______ I heard someone cry “ Earthquake!” loudly outside.
I am considering buying ______ used car for its lower price.
It is clever ________ you ________ me work out the maths problem.
There is ________ in the classroom, so we can see ________.
We should prevent the children ________ the bad programmes.