The Isoelectronic Point of an Amino Acid Is the Point $\alpha$

The isoelectronic point of an amino acid is the point where

A) the pK_a of the $\alpha$ -carboxylic acid is the same as the pK_a of the $\alpha$ -amino group.
B) the $\alpha$ -carboxylic acid is protonated and the $\alpha$ -amino group is unprotonated.
C) the solubility of the amino acid is maximized.
D) the $\alpha$ -carboxylic acid and the $\alpha$ -amino group are both half protonated.
E) the amino acid carries no net electrical charge.