The intensity of an activity refers to the
a. length of time it takes for an exercise or group of exercises to be complete.
b. number of times an exercise or group of exercises is performed within an certain time frame.
c. amount of energy exerted while performing an exercise.
d. classification of exercise.

C. The amount of energy exerted while performing an exercise. 

People who utilizes the progression principle will ensure their exercise program increases in frequency, intensity, time, and type. the changes need to happen gradually in order to ensure they make gains in their fitness program. a person who doesn't utilize the progression principle can still gain health benefits from exercising, but will not continue to improve in cardiovascular fitness.

What are the items? ?