A group of scientists studying calcium metabolism in laboratory rats discovered that removing the rats’ parathyroid glands resulted in the rats’ having substantially lower than normal levels of calcium in their blood. This discovery led the scientists to hypothesize that the function of the parathyroid gland is to regulate the level of calcium in the blood by raising that level when if falls below the normal range. In a further experiment, the scientists removed not only the parathyroid gland but also the adrenal gland from rats. They made the surprising discovery that the level of calcium in the rats’ blood decreased much less sharply than when the parathyroid gland alone was removed.
Which one of the following, if true, explains the surprising discovery in a way most consistent with the scientists’ hypothesis ？（）
A．The adrenal gland acts to lower the level of calcium in the blood.
B．The adrenal gland and the parathyroid gland play the same role in regulating calcium blood levels.
C．The absence of a parathyroid gland causes the adrenal gland to increase the level of calcium in the blood.
D．If the adrenal gland, and no other gland, of a rat were removed, the rat’s calcium level would remain stable.
E．The only function of the parathyroid gland is to regulate the level of calcium in the blood.