The boy often forgets        his homework. So his teacher is angry

 When I walked past his house, I heard him _______.

 ______ to forget is a good way to solve our problems and we can also deal with them by ______ of something worse.

 It is better for you to walk down the stairs instead of ________ the elevator.

Don’t throw away the waste paper. It needs        so that it can be reused.