A hormone produced by the beta cells of the islets of Langerhans in the pancreas is called:

A) glucagon.
B) insulin.
C) melatonin.
D) calcitonin.

Insulin is produced by the beta cells in the pancreas and is responsible for regulating blood sugar levels by facilitating the uptake of glucose by cells. Glucagon, produced by the alpha cells in the pancreas, raises blood sugar levels by stimulating the breakdown of glycogen in the liver. Melatonin is a hormone produced by the pineal gland that regulates sleep and wake cycles. Calcitonin is produced by the thyroid gland and is involved in regulating calcium levels in the blood.