How big a problem is the pollution caused by cities?
城市造成的污染是个多么严重的问题？
The pollution caused by autos is the most important problem at present.
由汽车造成的污染是目前最重要的问题。
One of the major reasons behind this spending spree is the country's serious air pollution problem, which is largely caused by dirty coal power plants.
中国之所以愿意大掏腰包，主要原因之一是中国的空气污染十分严重，而这很大程度上是由于高污染的燃煤电厂导致的。
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