Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks.

Everyone wants to reduce pollution. But the pollution problem is as complicated as it is serious. It is complicated because much pollution is caused by things that benefit people. (21) __________, exhaust from automobiles causes a large percentage of air pollution. But the automobile provides transportation (22) __________ millions of people factories discharge much of the material that pollutes air and water, but factories give (23) __________ to a large number of people. Thus, to end or greatly reduce pollution immediately, people would have to (24) __________ using many things that benefit them. Most people do not want to do that, of course. But pollution can be gradually reduced in several ways, scientists and engineers can work to find ways to lessen the amount of pollution that such things as automobiles and factories cause. Governments can pass and enforce laws (25) __________ require businesses and individuals to stop, or cut down on certain polluting activities.

(21) __________, exhaust from automobiles causes a large percentage of air pollution.