People from the High-Altitude Highlands of South America Develop Larger

People from the high-altitude highlands of South America develop larger lung capacity in order to process more oxygen. Why does this same lung capacity not reverse if these individuals move to a lower altitude?

A) Altitude acclimatization is permanent.
B) Advantageous phenotypic traits are genetically determined.
C) This trait is a variation that results from only genetic factors.
D) This is a permanent developmental adaptation.