Answer:
A. He has a large buildup of lactate in his muscles.
Explanation:
Ideally, our body cells, via the process of aerobic cellular respiration, need oxygen to break down glucose. However, during rigorous activities or exercises, oxygen can become unavailable for use. In this case, the body cells switch to use another way of cellular respiration, which is FERMENTATION.
This fermentation causes a build up of lactic acid/lactate (its byproduct) in the cells of the muscles being used, which physically causes a burning and aching sensation just as the case of this hiker. The climbing of the steep path is causing causing the fast usage of oxygen in his cells.
