Why Did the United States Become Involved in the Conflict

Why did the United States become involved in the conflict between North and South Vietnam in 1961?

A) to stop the spread of communism from North Vietnam to South Vietnam
B) to gain control of valuable natural resources in South Vietnam
C) to honor obligations to Russia from the treaty ending World War II
D) to retaliate for North Vietnam's involvement in the Bay of Pigs operation
E) to stop South Vietnam's bloody takeover of North Vietnam