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.

Air pollution occurs when wastes dirty the air. People produce most of the (61) ______ that cause air pollution. Such wastes can be in the form of gases or particles of solid or liquid matter. These substances result chiefly from burning (62) _______ to power motor vehicles and to heat buildings. Industrial processes and the (63) _______ of garbage also contribute to air pollution. Natural pollutants (impurities) include dust, pollen, soil particles, and naturally occurring gases.

The rapid growth of population and industry and the increasing use of automobiles and airplanes have made air pollution a (64) _______ problem. The air we breathe has become so filled with pollutants that it can cause (65) _______ problems. Polluted air also harms plants, animals, building materials, and fabrics. In addition, it causes damage by altering the earth’s atmosphere.

They_________their service up to now. A. didn’t do B. haven’t done C. don’t do D. aren’t doing