Both A and B is the expected answer, I expect.
I'm not sure it is either. The real reason has nothing to do with ideology. Economics and political influence usually are much more important. I will use two examples. For 11 months, the Soviet Union blocked West Berlin's food supply (June 1948 to May 1949)
The United States and Great Britain finally said (in effect) that they had had enough. They amassed a huge armada of air craft and flew supplies into West Berlin. The Soviets backed down and let commerce go on as usual.
There were other events that annoyed both sides. NATO was a huge thorn in the Soviet's side. It kept on creeping ever closer to the Soviet Union. The Soviets responded with the Warsaw pack.
The result was a standoff. These were the events of the Cold War. No fighting: just a lot of positioning and troop movement.