In a Titration of Monoprotic Acids and Bases,there Is a Large

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 at which pH = pK_b of the base.