Correct answer:
C. The United States and the Soviet Union developed the ability to completely destroy each other.
As the Cold War continued, the two superpowers (US and USSR) kept escalating their weapons capabilities and stockpiles. It got to the point that if the two sides did plunge into war, they would face mutually assured destruction -- abbreviated as M.A.D. It seemed the Cold War had driven the two powers mad (crazy) in their efforts to arm themselves against each other.
Eventually the US and USSR would pursue policies of detente, which included pledges to reduce their nuclear arsenals. The arms race and solving the arms race were constant issues affecting the Cold War.