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Air pollution is a serious problem in many cities. Motor vehicles, factories and other sources create so much air pollution that it may hang in the air like dirty fog. Air pollution threatens the health of the people who live in cities. City wastes cause water pollution when they are poured into the waterways. These wastes kill fish and make some areas unfit for swimming. In addition, many large cities have difficulties in disposing of their garbage. The amount of garbage grows each year, but places to put it are quickly filling up. Citizens, governments, industries, scientists, and business people must work together in different ways to gradually reduce pollution. For example, most cities have introduced recycling programs, which cut down on garbage and the space needed to store it. Residents separate recyclable products from garbage, and city governments provide drop-off cities or pick up the products from homes.

A. Air pollution is a serious problem in all cities.

B. Air pollution is a serious problem in countries.