Michael had to write a paper on the formation of red blood cells. In addition to the circulatory system, what other body system should he research? a. Skeletal system
c. Endocrine system
b. Immune system
d. Muscular system

The correct answer is d

The answer is D- muscular system

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)