What is appetite? a basal metabolic rate a desire for food based on emotional factors a feeling of discomfort caused by nutrients a feeling of hunger

I believe the answer is a feeling of hunger

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.

Ican only answer your first question, a machine is like a human body because it's a little similar. they both almost work like the same.