All living things need air to live. People need clean air to breathe. But sometimes people make the air dirty. They drive so many cars that the fumes from the cars pollute the air.
Fumes are made when gasoline is burned. The fumes that come out of cars, buses and trucks can pollute the air.
Environmental engineers are people who work to protect the environment. One of their jobs is to design or plan cars that will not pollute the air. Some time ago environmental engineers designed an object for cars that removes harmful pollution. Now cars are causing less pollution than they once did, because environmental engineers worked hard to protect the environment.
Why do cars cause less pollution than before
Because ______.