Our teacher says that the classroom must __________ every day.

It is best to eat three meals a day; otherwise, it is possible____ your health will suffer later in life

Two thirds of the earth _____ covered with water. [ ]

The notice came around two o’clock in the afternoon _______ the meeting would be put off.

How difficult math is! I really want to drop it ! —_________you come on，you will do it better．