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
-One of the greatest contributing factors to the intensity of the conflict in Vietnam was

A) the expansion of collective security agreements.
B) the rise of a Vietnamese nationalist movement.
C) a series of oil crises in the region.
D) the failure of efforts at peaceful, mutual coexistence.