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BEIJING, CHINA - NOVEMBER 20: A Chinese man wears a mask to protect against pollution as he uses his smartphone on a hazy day outside the Forbidden City November 20, 2014 in Beijing, China. United States President Barack Obama and China's president Xi Jinping agreed on a plan to limit carbon emissions by their countries which are the world's two biggest polluters at a summit in Beijing last week. (Photo by Kevin Frayer/Getty Images)

BEIJING, CHINA - NOVEMBER 20:  A Chinese man wears a mask to protect against pollution as he uses his smartphone on a hazy day outside the Forbidden City November 20, 2014 in Beijing, China. United States President Barack Obama and China's president Xi Jinping agreed on a plan to limit carbon emissions by their countries which are the world's two biggest polluters at a summit in Beijing last week.  (Photo by Kevin Frayer/Getty Images)