The correct anwser would be climate, because the great potato famine is part of the climate, and was a deciding factor as to why they should leave. I hope you find this helpful, and good luck! :P
Factors which caused an Increase in Air Pollution in China:
A.) Coal Combustion
B.) Slash and Burn
C.) Excessive disposal of Garbage into Open Area
Hope this helps!
The western coast of the U.S