Ancient scientists proposed many theories based not only on scientific observations, but also on religion and philosophy. These scientists, including Aristotle and Ptolemy, had developed an early model of the solar system. This model appeared to describe the motions they observed in the heavens. Which of the following objects is at the center of this model? A. sun
B. star
C. moon
D. Earth

You exhale co2 which is produced during cellular respiration.  co2 combines with the water in your blood's plasma to make up one half of the body's most important buffer pair, carbonic acid.  the more physical activity you engage in, the more co2 your body is producing.  you can see this by putting some of the cabbage indicator in a glass and then blowing bubbles into it through a straw.  can you see a change in the color of the indicator?