Why Is the P_O2 of the Blood Leaving Pulmonary Capillaries

Why is the P_O2 of the blood leaving pulmonary capillaries 104 mm Hg, but the blood leaving the lungs to enter the left atrium is 95 mm Hg?

A) Some O₂ diffuses out of the blood as it travels through the pulmonary veins.
B) Some O₂ is incorporated into bicarbonate ion, removing it from solution and reducing P_O2.
C) Some CO₂ in the alveoli is mixed with oxygenated blood from alveolar circulation.
D) Deoxygenated blood from bronchial circulation is mixed with blood from oxygenated alveolar circulation.