B. It prevented either country from attacking the other because of the
threat of an immediate return attack.
Explanation:
This is because both sides knew that if one did something such as nuke the other, they would be nuked right back. Therefore they both or mutually are aware that any offensive act against the other would lead to an equally destructive defensive act. This affected the Cold War as it made it a "war" without direct fighting. Sure there were places where both sides were indirectly fighting (ex. communism v containment in Korea and Vietnam) but overall there was no direct conflict as both sides knew it would cause an intense amount of damage to each side. (if you need any clarification let me know, hope this helps)
