Based on the following,which is true regarding the acid/base properties of guanine?

A)at physiological pH,guanine will be positively charged
B)at physiological pH,guanine will be negatively charged
C)at physiological pH,guanine will be neutral
D)at physiological pH,guanine will be a very effective buffer against base
E)at physiological pH,guanine will be a very effective buffer against acid