Genocide is defined as mass murder. It is done to eradicate a race, an ethnic group, or a nation.
C. The Mongol Sack of Baghdad in 1258 is an example of genocide.
Mongol soldier killed the citizens of Baghdad regardless if said citizen is a man, a woman, or a child. It was said that the death toll ranged from 200,000 to 1 million people of Baghdad resulting to it being depopulated.
They moved to different places that accepted blacks and started secret schools taught by willing whites.
The answer is C. The atmosphere is essentially the surroundings of anything. In the case of the earth, however, it is gasses.