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.

More oxygen is needed during exercise by the body as compared to normal condition because our body needs energy which is only generated in the process of cellular respiration. In cellular respiration, glucose molecule is broken down by mitochondria of the cell and energy is generated in the form of adenine tri phosphate (ATP) which is used for performing muscular activity. So that's why oxygen requirement is more during exercise.