If a person usually works or plays at a ____ intensity, but for a long duration, they have developed an ability to maintain aerobic energy production. As long as a person's body can meet its energy demands in this oxygen-rich mode, it will not convert to anaerobic energy production.

Marathon runners, serious joggers, distance swimmers, aerobic dancers, and bikers can perform because of their highly developed aerobic fitness. The cardiorespiratory systems of these aerobically fit people take in, transport, and use oxygen in the most efficient manner possible.
a) Low
b) Moderate
c) High
d) Variable