Which of the Following Is True About the Vietnam War

Which of the following is true about the Vietnam War?

A) U.S.combat forces left Vietnam in 1973.
B) North Vietnamese forces took over the country in 1975.
C) It was the most painful and costly application of the containment doctrine.
D) U.S.forces were technically superior in combat to the communist fighters,although the war had no front lines and few set battles.
E) All these answers are correct.