The Imidazole Side Chain of Histidine Can Function as Either

The imidazole side chain of histidine can function as either a general acid catalyst or a general base catalyst because
(I) the imidazole group has a pK_a in the physiological pH range.
(II) in the physiological pH range,the nitrogen in the ring can be easily protonated/deprotonated.
(III) one guanidino group is protonated,and the other is deprotonated at physiological pH.
(IV) the imidazole group is a strong reducing agent at physiological pH.

A) I,II
B) I
C) II,IV
D) II,III
E) IV