In a Titration of Monoprotic Acids and Bases, There Is

In a titration of monoprotic acids and bases, there is a large change in pH ________

A) at the point where pH = pK_a of the acid.
B) when the volume of acid is exactly equal to the volume of base.
C) when the concentration of acid is exactly equal to the concentration of base.
D) when the number of moles of acid is exactly equal to the number of moles of base.
E) at the point where pH =pK_b of the base.