Please answer the following question: Write a multi-choice question for the following article, with the given choices and answer: Article: While students in Hainan are quite used to clear skies, Beijing teenagers are not so lucky. As another warm winter approaches, the city can expect the normal clouds of smoke caused by air pollution. But things could start to get better soon. The government is co-operating with a US- based environmental protection agency to update existing buses and trucks with clean fuel technology. The new technology could reduce air pollutants in existing diesel vehicles ( ) by 40 per cent. The programme will begin by testing buses in Beijing to see if the technology can be applied to them. "We encourage the development of public transportation. But at the same time we need to reduce pollution from them," said an official. Efforts are being made to improve the capital's environment with tighter controls on emissions . Some heavily polluting factories and construction sites, such as those owned by the steel giant Shougang Group, have been asked to cut production in November and December or be closed. Beijing was the third polluted city in the world at the end of last century, according to the UN. But thanks to recent measures, the capital has made some progress. Last year 224 clear days were rated as having good air quality. In 1998 the air quality index gave just 100 days as good. "I am glad to see an improvement," said a Senior 1 boy living in the northwest of Beijing. "Compared to other places, the air quality of Beijing is still worrying though. I hate the pollution. Once I was riding my bike in the morning when I almost had a traffic accident because I couldn't see a car only metres away from me in frog." In early October, the skies were covered by such a thick fog that a display show by the visiting French air force was called off. Rapid development, industry, traffic fumes and sandstorms from the desert all contribute to the city's bad air. Options: A a programme to improve Beijing's air quality B progress made in Beijing's air quality C Beijing's air pollution D the difference between Hainan and Beijing Answer: C Beijing's air pollution Question:
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The passage is mainly about _ ?