Answer:
The right answer is D.
Step-by-step explanation:
We must remember the historical world context. There was a contest over global influence between the United States and the Soviet Union. America went to war in Korea in 1950 because it wanted to oppose what it perceived as a deliberate move of Communist expansion in Asia. In principle, it got involved in Vietnam for the same reason: to stop Communist advance in Asia, causing other countries to fall in the hands of Moscow-backed Communists (this is what we call the "domino theory").