_____________ (应当注意的是) that pollution has become one of the thorniest problems in many big cities.

Many Americans want to have their homes outside the cities and their jobs in the cities.

Please identify the cause in each of the following situation. In recent decades, many people have poured into the cities and the cities have grown so large that now about 50% of the Earth's population lives in urban areas. A: The cities have grown so large. B: Many people have poured into the cities. C: Now about 50% of the Earth's population lives in urban areas. D: None of them.

The activities available to people in rural areas are much morethan to_______ people in big cities. (limit)

How many cities<br/>will have air quality monitoring systems installed by the end of this<br/>year?() A: 42 cities. B: 220 cities. C: 150 cities. D: 262 cities.