Which outcome is a psychological effect of stress? a. depression b. heart disease c. obesity d. fatigue

A.  Depression
the other choices would be considered physiological signs of stress.

Your answer is  d.)relationship between pressure and air volume and led to the understanding of how respiration works

Well if you pick a machine to work on and that's gonna be your exercise if you don't stick to that machine your muscle strength your muscle mass rather will change because you haven't been doing it you change something in your exercise routine which changes your body's form of dealing with things for example you might become weaker or stronger depending on what kind of exercises you do in whether or not the machine you picked actually your muscles grow.