1. using carbo-loading to increase the store of glycogen seems to benefit only those athletes who engage in aerobic activities that are a. between 50 and 80 minutes. b. more than 90 minutes. c. less than 20 minutes. d. between 30 and 60 minutes.

Carbo-loading is a technique commonly used by endurance athletes who are involved in activities that last more than 90 minutes. It is used as a strategy to increase the amount of fuel stored inside the muscles for improvements in athletic performance. It is usually done a week before a high-endurance activity. 

Carbohydrates are the body's first source of energy taken from food. When a person intakes carbohydrates, the body breaks it down in the form of sugar where it transfers to the bloodstream. Sugar that is not utilized or used will be stored in the liver and muscles in the form of glycogen. During endurance races, when carbohydrates are no longer available, the body uses this stored energy to help fuel the rest of the activity.