Please answer the following question: Read the following article and select the best answer. Article: XI'AN - China has introduced various emergency responses and long-term measures against air pollution in the central and eastern regions. The air quality index (AQI) for Xi'an reached 500 on Wednesday afternoon, at the top of the AQI scale, according to the national air quality monitoring website. Xi'an weather forecasters predict the smog will continue for the next week and air pollution may worsen. "I have to keep buying masks for my family," said Liu, buying masks at a pharmacy near her home in downtown Xi'an. "This is the only thing we can do to protect ourselves." On Wednesday, a city government emergency response plan was put into action, _ all construction and taking at least 50 percent of government vehicles off the road. Power plants must limit their output and reduce emissions. Nearly half the country, more than 100 cities in 20 provinces, has suffered from smog since last December. Many have taken action, including experimenting with artificial methods to reduce smog, limiting vehicle use and fining polluters. Assessments based on GDP alone were abandoned last month and more emphasis has been put on public well-being and the environment. Whether the move will be effective in forcing local governments consider environmental issues over economic growth remains to be seen. Environmental protection professionals are doubtful about the immediacy of the effects of these measures. Xiao Hang, an urban researcher at the Chinese Academy of Sciences, said it took London 50 years to lose the moniker "foggy London town", and Los Angeles still struggles to control its photochemical smog. "We can't ignore environmental problems during the development process and China has a long way to go," he said. Question: What would be the best title for the article? - AQI for Xi'an reached 500 - Smog crisis tries the limits of people's patience - Smoggy Chinese cities to clear the air - Dust masks become hot sellers
A:
Smoggy Chinese cities to clear the air