Carbon dioxide levels in the blood and cerebrospinal fluid affect blood pH. This enables the organism to sense a disturbance in gas levels as ________. A. the brain directly measures and monitors oxygen levels and causes breathing changes accordingly. B. the brain alters the pH of the cerebrospinal fluid to force the animal to retain more or less carbon dioxide. C. the medulla oblongata, which is in contact with cerebrospinal fluid, monitors pH and uses this measure to control breathing. D. stretch receptors in the lungs cause the medulla oblongata to speed up or slow breathing.