The traditional view is ______ we sleep because our brain is “programmed” to make us do so.

3. The students will ________ for the Christmas vacation next week.

It was in 1969 ______ two Americans went to the moon by spaceship.

Your desk is covered with too many unnecessary things, including used papers and booksYou’d better ________ some of them

Growing population resulted in growing rubbish.—There is ______that rubbish will become the major threat to human beings in future.