The adjoining illustration shows several oxygen-dissociation curves. Assuming that curve 3 corresponds to hemoglobin with physiological concentrations of CO2 and 2.3-BPG at pH 7.0. Which of the curves represents each of the following perturbations?
a) Decrease in CO2
b) Increase in 2,3-BPG
c) Increase in pH
d) Loss of quaternary structure