Oxygen-free hemoglobin absorbs 10 times more light at 690 nm than fully oxygenated hemoglobin. If a sample of hemoglobin is found to absorb only 5 times as much light as fully oxygenated hemoglobin, what percentage of the hemoglobin molecules have oxygen bound to them? (Assume that each hemoglobin molecule is either fully bound or fully unbound.)

a. 50%

b. 5%

c. 690%

d. none of these

e. 10%