After Blood Is "Used" by Cells, in Other Words They've

After blood is "used" by cells, in other words they've taken the oxygen, where does it go?

A) Back to the lungs for more oxygen
B) Back to the heart and then on to the lungs for more oxygen
C) Back to the heart to return to the body for more oxygen
D) Back to the capillary beds for more oxygen
E) Back to the brain, where the oxygen level is monitored