While a Runner Rests Before a Race, the Hemoglobin in the Blood

While a runner rests before a race, the hemoglobin in the blood returning to her heart is 75% saturated with oxygen.While she is running, this percentage

A) stays the same because hemoglobin is usually 75% saturated with oxygen.
B) increases because she is breathing harder.
C) increases because her muscles need more oxygen.
D) decreases because the decreased PO₂ in her muscles makes it easier for additional oxygen to be released from the hemoglobin.
E) decreases because the partial pressure of oxygen in her alveoli is reduced below 40 mm Hg.