The answer is B. The atomic bombs that the United States dropped were the Fat Man and the Little Boy. They were dropped on Hiroshima and Nagasaki and killed about 146,000 in Hiroshima and 80,000 in Nagasaki. These bombs were made to push Japan to it's breaking point for it to surrender.
Thousands of slaves were freed by the operation of the Emancipation Proclamation as Union armies marched across the South. Lincoln issued the Emancipation Proclamation, which made ending slavery a war goal. The Emancipation Proclamation over time would reinforce the political liability of supporting the Confederacy.
During the Cold War, the United States was focused on the foreign policy of containment. This idea focuses on stopping the spread of communism on an international scale. This policy explains why the US got involved in Cuba and Vietnam.
In Cuba, Fidel Castro and his followers overthrew Batista and established a communist government. Along with this, Castro established a good trading relationship with the Soviet Union. This resulted in the US attempting to overthrow Castro through the Bay of Pigs Invasion. However, this mission failed and only increased Castro's power and influence in Cuba.
As for Vietnam, the US was always interested in what was happening there, as they were afraid the communist system in North Vietnam would spread to the democratic country of South Vietnam. This act gave President Lyndon B. Johnson the power to take all necessary measures to protect US interests in Vietnam and to increase military action if needed.