Final answer:
Cross-country runners are likely to predominantly rely on type I motor units for endurance.
Step-by-step explanation:
The athlete who is likely to predominantly rely on type I motor units is the cross-country runner. Type I motor units are slow-twitch muscle fibers that are resistant to fatigue and are used for activities that require endurance, such as long-distance running. Cross-country runners engage in prolonged periods of aerobic activity, making them heavily reliant on these type I motor units.
Learn more about type I motor units in athletes