What Is the Biggest Problem for People as a Result

What is the biggest problem for people as a result of global warming?

A) As soil moisture decreases, trees will die and wildfires are becoming more common, changing forests into savannas.
B) In a warmer world, tropical diseases such as malaria have been spreading to higher latitudes.
C) Because of high population density, land ownership, and political boundaries, people cannot migrate as freely as they once could.
D) Unless people build expensive dams, there is less water available for irrigation.