The answer to fill in the blank would be D) Modifier because a subject that's complete has to have of course the subject and a modifier to modify the subject or sometimes a verb.
The Tet offensive proved that not only did the Viet Cong have the desire to fight back, but also the ability. It killed many US soldiers and set the war strategies back a bit. This added deeper conflict and dread to American fighters as well as American civilians.
