Đọc kỹ đoạn văn sau và chọn phương án đúng (ứng với A, hoặc B, C, D) cho mỗi câutừ 1 đến 4. AIR POLLUTION Pollution is the process of making something dirty. Sometimes, the air becomes dirty from natural causes. If a volcano erupts, it will fill the air with gases, pieces of ash and smoke. People often cause the air to become polluted. When garbage becomes old, a gas called methane fills the air. Cars fill the air with fumes. A lot of factories emit poisons into the air. Weapons of war also add toxic gases into the air. To decrease air pollution, air quality standards, like Clean Air Act, were enacted in 1963 and then some pollutants have been reduced.