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