<em>B. Soviet building of the Berlin Wall.</em>
Explanation:
After World War II ended, Germany got split up amongst the Allied powers. The United States, France, and Great Britain got West Germany, while the Soviet Union got East Germany.
The Soviet Union centered around communism, so East Germany had a communist government. Many Germans realized they did not want to obey and live under this government and wanted to flee. East Germans started leaving and moving to West Germany.
Over time, East Germany started to become fearful that too many people were leaving and decided they needed to do something about it. They built the Berlin Wall that was heavily guarded and hundreds of people ended up dying trying to get through.
This was the first cause of tension between the Soviet Union and President Kennedy. Both John F. Kennedy and Ronald Reagan gave speeches about the wall and how horrible it was.
