A series of gas mixtures is inhaled by a healthy subject. Which one of the following gases would diffuse most slowly from the lungs into the blood? A) CO2 at PCO2 of 60 mmHg B) CO at PCO2 of 0.5 mmHg C) O2 at PO2 of 130 mmHg D) O2 at PO2 of 150 mmHg Explain your answer.