Reading Comprehension

Read the text and answer the question.

Some athletes train at high altitude to improve their performance at sea level. In thin mountain air, the body receives less oxygen with each breath. To cope, it gradually produces more red blood cells, which carry oxygen to the muscles. When the athlete later competes at lower elevation, where air is richer in oxygen, this boosted blood supply can give a temporary edge. However, the benefit fades within a few weeks once training returns to normal conditions. Coaches must therefore time altitude camps carefully, balancing the advantage against the difficulty of training hard in air that leaves athletes quickly tired.