Why did china export so much during the 15th century?

Eventually the Europeans, for reasons of their own, became reluctant to continue shipping so much silver to East Asia. This is largely because they preferred to hoard the silver so that they could use it to pay mercenaries in their ongoing wars. They started looking for something else to export to China and found that they were in a real bind because there were very few things that they produce more efficiently than the Chinese could. The thing that eventually filled the gap left when the Europeans tried to cut back on their silver shipments was opium.  

Opium served a whole series of functions for the British in particular. It helped make their new colony in India profitable by providing a very ready revenue source. It saved on the silver that they no longer wanted to ship and, of course, the story of the opium trade to China then gets us into a whole different period of world history and different kinds of links between China and the outside world.  

a.) are both appointed for life