The Following Questions Refer to the Excerpt Below

The following questions refer to the excerpt below.
"Why are we in South Viet-Nam? We are there because we have a promise to keep. Since 1954 every American President has offered support to the people of South Viet-Nam. We have helped to build,and we have helped to defend. Thus,over many years,we have made a national pledge to help South Viet-Nam defend its independence…We are also there because there are great stakes in the balance. Let no one think for a moment that retreat from Viet-Nam would bring an end to conflict. The battle would be renewed in one country and then another."
President Lyndon B.Johnson,Speech at Johns Hopkins University,1965
-The most significant concern of the United States government in its involvement in the conflict described by President Johnson was

A) stopping the potential spread of Communist influence.
B) securing American economic security.
C) protecting America's position as the most powerful nation on earth.
D) developing new alliances in Southeast Asia.