Chọn một đáp án trong số A, B, C hoặc D điền vào mỗi chỗ trống để hoàn thành đoạn văn sau:       

     Air pollution, or dirty air, is (41)________ by many things. Great clouds of smoke come from factories chimneys. Cars send smoke and fumes into the air. Burning waste fills the air (42)_______ bits of dirt. Even burning leaves adds smoke to the air. Smoke of all kinds is bad to breathe and can do great harm to the body. (43)_______ countries have made laws to limit the amount of fumes cars can exhaust into the air. They also set limits on pollution from power plants and factories. Although scientists have not yet been able to stop air pollution, they have found ways to make cars and factories burn fuel more cleanly. People are also more careful than they once were. For example, they have stopped burning fallen leaves in the (44) ______. We still have a long way to go. But if we all work together, we can (45) _________the problem.

          Air pollution, or dirty air, is (41)________ by many things. lem.