During the Cold War Era, What Was the Main Reason

During the Cold War era, what was the main reason why were the United States and the Soviet Union reluctant to engage in direct military conflict?

A) There was a great potential for global nuclear destruction.
B) East and West German had been re-unified.
C) Nonaligned nations were pressuring the United States and the Soviet Union to avoid fighting.
D) There were greatly increased tensions in the Middle East.