Sometimes it is necessary to (17)........... our habits. Saving energy is one illustration. If we can (18) .......... more energy, such as water and electricity, we can help keep the natural environment safe. Saving water, for example, helps preserve out rivers and reservoirs. If we usually take 10- minute showers, in this case we should try to take shorter showers. When we go out, we (19) .......... remember to shut off electronic like televisions and computers. Turing (20)......... the lights is another example of saving electricity. In short, remembering to help the environment, such as by saving energy, is one of our most necessary responsibilities. 17. A. exchange B. turn C. change D. make 18. A. help B. cut C. saving D. save 19. A. must B. may C. had D. should 20. A. on B. in C. without D. off