The concave disc shape of red blood cells Maximizes the surface area of the cell , allowing for rapid movement of oxygen or carbon dioxide to and form the hemoglobin within the cell.
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13
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Your answer is A. 13 Hope this helps!
Parietal cells within the stomach lining secrete hydrochloric acid that lowers the pH of the stomach. A low pH (1.5 to 2) activates pepsin. Acetylcholine, gastrin, and histamine stimulate the proton pump in parietal cells to release hydrogen ions and decrease pH.
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These are things that won't be going away. Natural gasses and fossil fuels will likely run out someday or become much harder to find. When you use these sources of energy, they won't be going away and therefore will be sustainable because we can always continue using them.
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