Which of the following statements about calcitonin is true:
a) Calcitonin secreted by parafollicular cells of the mammalian thyroid is a single-chain peptide hormone with 32 amino acids
b) Effects of calcitonin are to lower serum calcium and phosphate by acting on bones and kidneys.
c) Calcitonin inhibits osteoclastic bone resorption.
d) All of the above
