What is the primary energy system to fuel cycling at 70% of vo2 max for 15 minuets?

The primary energy system to fuel cycling for 15 minutes is THE AEROBIC GLYCOLYSIS SYSTEM.
There are different energy systems for fueling different types of physical exercise, the type of system that is used depend on the duration of the physical exercise. The energy system that exist are anaerobic glycolysis, aerobic glycolysis, phosphagem system and fatty acid oxidation. The aerobic glycolysis energy system relies on the complete breakdown of glucose. The pyruvate which is the end product of glucose oxidation is further broken down to acetyl coA, which then enters into the citric acid cycle. The aerobic glycolysis energy system is used for physical exercises that last between three to twenty minutes.