单项选择题

From 1946 until 1954, the Vietnamese had struggled for their independence from France during the First Indochina War. At the end of this war, the country was temporarily divided into North and South Vietnam. North Vietnam came under the control of the Vietnamese Communists who had opposed France and who aimed for a unified Vietnam under Communist rule. The South was controlled by Vietnamese who had collaborated with the French.
The United States became involved in Vietnam because it believed that if all of the country fell under a Communist government, Communism would spread throughout Southeast Asia and beyond. This belief was known as the "domino (多米诺骨牌)theory." The U.S. government, therefore, supported the South Vietnamese government. This government’s policies led to rebellion in the South.
In 1965 the United States sent in troops to prevent the South Vietnamese government from collapsing. Finally, however, the United States failed to achieve its goal. and in 1975 Vietnam was reunified under Communist control: in 1976 it officially became the Socialist Republic of Vietnam. During the conflict, approximately 3 to 4 million Vietnamese on both sides were. killed. More than 58.000 Americans lost their lives.

In the sentence "The South was controlled by Vietnamese who had collaborated with the French. ", "collaborate" means（）

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