Choose the Correct Answer from the List Below

Choose the correct answer from the list below. Not all of the answers will be used.
-____________: The amino acid with a pKa near neutral pH.

A) L amino acids
B) water
C) protons
D) zwitterions
E) leucine
F) serine
G) tyrosine
H) cysteine
I) glutamate
J) histidine
K) proline
L) asparagine
M) D amino acids