BEIJING--Heavy haze   still covered north and east China on Saturday, with highways closed and flights delayed or canceled  . The smoggy weather affected the country's land from north to south, including Beijing, Hebei, Shandong, Jiangsu, Shanghai, Zhejiang, Hunan, Guangxi and Guangdong, with visibility less than 50 meters in some areas. Cities like Shanghai and Nanjing have been enveloped in the grey sky for the past several days. Many walkers in the street had to wear masks. In the Lukou airport in Nanjing, Jiangsu Province, more than 60 inbound and outbound flights were canceled on Saturday because of the hazy, which also forced primary and middle schools and kindergartens to stop classes on Thursday and Friday in Nanjing. The Ministry of Environmental Protection said earlier this week that there were several main reasons behind the widespread haze: unfavorable weather conditions making it difficult for pollutants to diffuse  , motor vehicle exhaust  , coal use for winter heating. The Chinese government now plans to clean up the air by cutting coal use, closing some factories and promoting cleaner production. "We must face up to the realities," said Yan Naiqiang, a professor of environmental science at Shanghai Jiao Tong University. "The treatment of smog will be a long battle." Which of the following is  _  a reason for the heavy haze?
Bad weather conditions. Too many flights. Motor vehicle exhaust. Coal use.