Many athletes use altitude training to improve their performance. They travel to areas of high altitude where there is less oxygen in the air. The body adapts to the stress of less oxygen by becoming more efficient at transporting and using oxygen. What are athletes who train in high altitudes most likely trying to enhance?

Its A

Explanation:

Energy Production in the Mitochondria because of the lack of less oxygen you would want to build up more ways to get more energy production going

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.