The Figure Below Is from an Experiment on Athletes

Multiple Choice

The figure below is from an experiment on athletes. The "live high, train low" group lived for 18 days in rooms with a simulated altitude of 3,000 m, while training (exercising) at 1,200 m. Use this information and this figure to answer the following questions.
-The scientists who conducted this experiment were testing the hypothesis that living at high altitudes will boost red blood cell production, which can then affect stamina when training and competing at lower altitudes. Why did they measure the athletes' aerobic capacity, which is the maximum amount of oxygen used in one minute per kilogram of body weight?

A) Red blood cells carry oxygen, which is needed for muscle function. An increased ability to take in oxygen should correspond to more red blood cells.
B) Oxygen is needed for muscle function. More oxygen is needed to do the same amount of work at high altitude compared to low altitude.
C) Athletes with high production of red blood cells need less oxygen than those with a low production of red blood cells.