The correct answer is A. The Cold War began when the USSR refused to pull its troops out of Eastern Europe. Following the end of World War II, tensions between the Soviet Union and the Western Allies, particularly the United States, began to escalate. The Soviet Union, led by Joseph Stalin, sought to spread communism throughout the world, while the United States sought to contain communism and promote democracy. The Soviet Union's refusal to withdraw its troops from Eastern Europe, combined with the United States' policy of containment, led to an arms race and a period of heightened tension and competition between the two superpowers, which became known as the Cold War. The United Nations was organized in 1945, after the end of World War II, but it was not the cause of the Cold War. Similarly, the division of Germany into East and West Germany did not cause the Cold War, but rather was a consequence of it.