Which factor played the GREATEST role in making World War I one of the deadliest conflicts in human history?

A) The use of atomic weapons in the Pacific and the genocide of the Holocaust in Europe.
B) New military technology like airplanes, machine guns, tanks, and poisonous gas made combat deadlier than ever before.
C) Improved transportation and shipping methods made getting troops and supplies from one place another easier than ever before.
D) Improved medical technology and methods reduced the effect of disease and improved survival rates for soldiers wounded in combat.