The answer is letter c.
It can be both natural and human source of pollution. There are times when the intense heat can
trigger a brush fire and can expand depending on how strong the wind
blows. It can be caused by humans. it could due to carelessness of people leaving
flammable materials unchecked. Or using too much fuel that when lit would cause
a brush fire that would expand into a wildfires. The smoke from it pollutes the air.
China.
The Silk Road is a trading route from the Middle East to China, and the ancient civilizaitons practiced ancestor worship. The Mandate of Heaven dictates that good weather and general prosperity are because of divine approval of the leader of China. If there is famine, invasion, or other natural disasters, it was interpreted as needing a new ruler, aka losing the mandate of heaven.
