The activity of an enzyme is close to zero at pH = 10 and above and high at pH = 6 and below. The activity of this protein probably depends upon the presence of an amino acid with R-group: A. that has a pKR = 8 and must be protonated for activity. B. that has a pKR = 6 and must be unprotonated for activity. C. that has a pKR = 10 and must be protonated for activity. D. that has a pKR = 4 and must be protonated for activity. E. that has a pKR = 8 and must be unprotonated for activity.