All of the cells in the body need oxygen. Hemoglobin molecules in red blood cells transport oxygen through the bloodstream. Oxygen is loaded onto hemoglobin molecules in the lungs and unloaded from the hemoglobin molecules in the tissues.
What drives the unloading of oxygen from hemoglobin molecules in the tissues?

A. the low partial pressure of oxygen in the tissues
B. the high partial pressure of carbon dioxide in the tissues
C. the low partial pressure of carbon dioxide in the tissues
D. the high partial pressure of oxygen in the tissues