You Spend Two Months in the Mountains of Peru at

You spend two months in the mountains of Peru at ~ 3500 m altitude. After returning to Toronto, you notice that you can run your regular 10 km route significantly more easily than before you left. What is the reason for this burst of fitness?

A) The time you spent at a high altitude resulted in an increased lung capacity and therefore extra oxygen intake. This extra oxygen stimulated the release of EP and in turn the production of red blood cells.
B) Due to lower oxygen concentrations at high altitudes, low blood oxygen levels stimulated release of EPO, which stimulated the production of red blood cells. This increased the oxygen binding capacity of your blood.
C) You were in the mountains, so you must have been hiking every day and got in better shape.
D) The time spent at a high altitude resulted in an increased active metabolic rate and therefore increased ATP production.