What Happened Two Years After the Peace Agreement Between the North

What happened two years after the peace agreement between the North Vietnamese and South Vietnamese?

A) The North Vietnamese invaded South Vietnam.
B) After 25 years of fighting,peace was finally achieved and the borders were opened to free trade.
C) The South Vietnamese invaded North Vietnam.
D) The United States pulled out of North Vietnam,abandoning all of the loyal North Vietnamese that had fought with the Americans during the war.