At a Physiological PH of 7

At a physiological pH of 7.4, amino acids exist as zwitterions.At a very low pH in an acid solution) , both the amine group and the carboxylic acid group on an amino acid are protonated.Which acid group is the stronger acid?

A) (-NH₂)
B) (-NH₃+)
C) (-COOH)
D) (-COO-)
E) (-H₃O+)