Why does the blood returning to the heart have very little oxygen in it?

a)the heart is not pumping oxygen fast enough
b)the cells throughout the body used the oxygen.
c)the heart did not receive enough oxygen from the lungs
d)the lungs breathed out too much oxygen

just put in mind that others aren't perfect. we all have problems that we feel makes us low of the low. but know that you are special and unique, that is what makes you out of this world : ). you might think and feel that others judge you, but mostly other people are scared if people judge them. just be happy, spend time with family, smile because that is the greatest medicine. a happy life. hope this : ))