Mr Smith Moved from a Low Altitude to a High

Mr. Smith moved from a low altitude to a high altitude and experienced an increase in RBCs. Which of the following caused this increase?

A) The lower temperature present at higher altitudes
B) The lower O₂ concentration seen at higher altitudes
C) The increased metabolic rate seen at higher altitudes
D) The increased CO₂ retention that occurs at high altitudes
E) Increased exercise