Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word for each of the blanks from 33 to 42.

Everyone wants to reduce pollution. But the pollution problem is (33)______ complicated as it is serious. It is complicated (34)______ much pollution is caused by things that benefit people. (35)______, exhaust from automobiles causes a large percentage of air pollution. But the automobile (36)______ transportation for millions of people. Factories discharge much (37)______ the material that pollutes the air and water, but factories give employment to a large number of people.

Thus, to end (38)_______ greatly reduce pollution immediately, people would have to (39)______ using many things that benefit them. Most of the people do not want to do that, of course. But pollution can be (40)______ reduced in several ways. Scientists and engineers can work to find ways to lessen the (41)______ of pollution that such things as automobiles and factories cause. Governments can pass and enforce laws that (42)______ businesses and traffic to stop, or to cut down on certain polluting activities.

Điền vào ô số 39

A. start 

B. continue

C. stop

D. go on