FILE - In this file photo taken Tuesday, Feb. 28, 2017, a passenger airliner flies past steam and white smoke emitted from China Huaneng Group's Beijing power plant that was the last coal-fired plant to shut down on March 18, 2017 as the Chinese capital convert to clean energy like thermal power. Researchers say Tuesday, April 25, 2017 that China's conversion of coal into natural gas could prevent tens of thousands of premature deaths annually. But there's a catch: It also could undermine efforts to rein in greenhouse gas emissions. (AP Photo/Andy Wong, File)

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