For the first question, the correct response is:
It feared that if Vietnam became communist then surrounding countries would quickly follow.
This is reflective of the domino theory, which suggested that the spread of communism in one country could lead to the spread in neighboring countries.
For the second question, the correct response is:
It convinced the United States to become involved in the war.
The domino theory was a significant factor that motivated U.S. involvement in the Vietnam War, as policymakers believed that failing to stop communism in Vietnam would lead to further communist expansion throughout Southeast Asia.