According to the FITT Principle, how intense should your cardiorespiratory endurance exercise be?

A. 60% to 85% of the maximum heart rate or RPE of 4

B. 50% to 65% of the maximum heart rate or RPE of 3

C. 35% to 50% of the maximum heart rate or RPE of 2

D. Above 85% of the maximum heart rate or RPE of 5