Read the following article and answer the question. Article: When Beijing is hit by smog, residents often joke that the Chinese capital needs to build giant fans to clear out the polluted air.Mountains to the north and west help trap smog in the city, and a strong wind from Mongolia is the surest way to ensure cleaner air. Beijing officials are now considering ways to help that process.Though giant fans are not on the menu, at least not yet, city planners are looking at ways to create corridors that encourage wind flow through the capital.The plan is inspired by research on urban heat islands, which shows that large buildings that block the wind are one of the causes of higher average temperatures in cities as opposed to those in the countryside. "This isn't controlling pollution.It's shifting the problem." Wang Bing, a Beijing-based author, wrote on Sina Weibo.But the plan has received some interest from residents who have grown tired of repeated periods of heavy pollution. Peng Yingdeng, an air pollution expert at the Beijing Municipal Research Institute of Environmental Protection, told the newspaper that the plan was more likely to require an adjustment of related laws to limit building heights and numbers in some areas.Similar proposals are under consideration in other major Chinese cities including Hangzhou, Nanjing, Shenyang and Wuhan. "This kind of work to search for ways to reduce pollution is really worth applauding," the state-run news agency Xinhua said in a commentary."If scientists confirm it is effective, then we should promote it on a much larger scale." Some experts have expressed doubts whether the plan would make a significant difference.Serious pollution is usually associated with periods of little or no wind, and air corridors do little good if the wind isn't blowing, Song Guojun, an environmental science professor told The Beijing News.Solving the air pollution problem mainly requires controlling pollution sources. Question: Why do people say "the Chinese capital needs to build giant fans"? Answer:

Options are:
(a). Because the weather in Beijing is very hot.
(b). Because they think air corridors are a good idea.
(c). Because Beijing is surrounded by mountains.
(d). Because the pollution is too serious.
Answer:
(d).