What Is the Mechanism by Which the Parathyroid Glands Maintain

What is the mechanism by which the parathyroid glands maintain calcium homeostasis?

A) They secrete parathyroid hormone (PTH) ,which promotes calcium storage in the intestines.
B) They produce a hormone that,with calcitonin,takes calcium from the bones and promotes calcium absorption by the intestines.
C) They stimulate the pituitary to produce PTH and calcitonin,which elevates serum calcium levels.
D) They secrete hormones that stimulate the production of calcium in the bones and small intestines when serum calcium levels fall below 4.5 mg/dl.