Ed is analyzing feedback mechanisms that maintain homeostasis in the human body. He follows the procedure that is listed below. (NOTE: Do not conduct this investigation at this time.) I. Invite a healthy test subject to participate in the investigation.
II. Measure and record the pulse and breathing rate of the subject.
III. Have the test subject exercise vigorously for 1 minute, such as by running in place.
IV. Repeat Step 2.

Why is Step 3 useful for Ed's purpose for the investigation?
a Exercise causes an increase in the pH of blood and other body fluids.
b Exercise increases the body's demand for food and nutrients.
c Exercise has no effect on the body's demand for oxygen, food, and nutrients.
d Exercise increases the body's demand for oxygen.