List the three main functions of the respiratory system

Which action best increases one’s stamina a) exercising when you feel like it b) saving so much that one like sleep c) refusing to exercise to save the energy needed to work d)exercising on a regular basis

As you sit in the library with your friend studying for your weekly quiz on the integumentary system, you come upon an article about the hazards of prolonged exposure to the sun's rays. your curious nature leads you to research the subject further. you're fascinated to learn that minimal exposure to the sun allows you skin to manufacture vitamin d, which is important to maintain homeostasis. you also read that although sunscreen can protect you from the ultraviolet (uv) rays of the sun, some medications can actually increase a person's sensitivity to the sunlight.

In this model, what do p, s, and a bases represent?