Refer to the Figure Showing the Hemoglobin-Oxygen Binding/dissociation Curve

Refer to the figure showing the hemoglobin-oxygen binding/dissociation curve. Which statement about hemoglobin's O₂-binding/dissociation curve is true?

A) At rest, our body's hemoglobin activity can be illustrated by only the upper one-fourth of its O₂-binding/dissociation curve.
B) A smaller decrease in PO₂ is required for hemoglobin to drop from 100 to 75% saturation than for hemoglobin to drop from 75 to 50% saturation.
C) Our blood normally carries a small reserve of O₂.
D) The steepest part of the O₂-binding/dissociation curve results in a small release of O₂ for a large drop in PO₂.
E) The PO₂ of our blood drops well below 40 mm Hg during light exertion.