What happened to vietnam after the u. s. pulled out of the war in 1973?

I believe the answer is A. 

D. The North defeated the South and incorporated it under a communist government.

Two months after the signing of the Vietnam peace agreement, the last U.S. combat troops leave South Vietnam as Hanoi frees the remaining American prisoners of war held in North Vietnam. America’s direct eight-year intervention in the Vietnam War was at an end. In Saigon, some 7,000 U.S. Department of Defense civilian employees remained behind to aid South Vietnam in conducting what looked to be a fierce and ongoing war with communist North Vietnam.