The rivers will become dirtier and dirtier _______ we take action to protect them.

The water was left running all night long. — It seemed that John ______. He was the last to do some washing.

In a position like this, one can ________ stand up _______ lie down . So it is very uncomfortable.

Can you think of some cases___________students know clearly he should listen to the teacher but can’t do that ?

has inproducing clones of cows and goats, and continues to research ways _______can benefit mankind