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In this picture taken on December 21, 2017, Ge Guoxiang (L) and Jin Hui pose for a portrait at their stall in the Tuanjiehu Tianyu market in Beijing. The hundreds of millions of migrants who moved from the countryside to the cities may have fuelled China's dramatic economic boom of recent decades. But many are no longer welcome in overcrowded Beijing, which seeks to cap its population at 23 million by 2020. / AFP PHOTO / Nicolas ASFOURI / TO GO WITH China-rights-economy-migration, FOCUS by Ryan MCMORROW and Joanna CHIU (Photo credit should read NICOLAS ASFOURI/AFP/Getty Images)

In this picture taken on December 21, 2017, Ge Guoxiang (L) and Jin Hui pose for a portrait at their stall in the Tuanjiehu Tianyu market in Beijing.
The hundreds of millions of migrants who moved from the countryside to the cities may have fuelled China's dramatic economic boom of recent decades. But many are no longer welcome in overcrowded Beijing, which seeks to cap its population at 23 million by 2020. / AFP PHOTO / Nicolas ASFOURI / TO GO WITH China-rights-economy-migration, FOCUS by Ryan MCMORROW and Joanna CHIU        (Photo credit should read NICOLAS ASFOURI/AFP/Getty Images)