(Solved): A study is performed to evaluate the diffusion capacity of the lungs with different gases. A line on ...

A study is performed to evaluate the diffusion capacity of the lungs with different gases. A line on the graph represents data obtained from a normal subject. Assuming that it takes <1 second for a red blood cell to pass through a pulmonary capillary, which of the following lines best shows the behavior of CO when experimentally given to a patient and transferred from the lung to the blood?

( I figured the answer is A, but I'd like to know why and what the other letters represent)