A is the answer. The first Bull Run was early in the war, and much of the North was convinced it would end quickly-people had even brought out chairs to watch the firefight. However, their visions were soon shattered.
B is wrong: the South had plenty of troops, and even if it did not, Bull Run was no indication.
C is wrong: Bull Run was a failure for the North, so it would not indicate to Lincoln that the North was overstocked on troops.
Similarly, D is wrong: the war actually would take longer.
