Correct Answer: A

Rationale: The main function of the digestive system is to break down food into nutrients through the process of digestion. Once the food is broken down, the nutrients are absorbed into the bloodstream and utilized by the body for various functions. Choice B, regulating body temperature, is incorrect as it is the function of the body's thermoregulatory system, not the digestive system. Choice C, transporting nutrients, is incorrect as the digestive system primarily focuses on breaking down and absorbing nutrients rather than transporting them. Choice D, producing hormones, is also incorrect as hormone production is mainly carried out by endocrine glands and not the digestive system.

Correct Answer: C

Rationale: Veins are the blood vessels that carry deoxygenated blood back to the heart, except for the pulmonary veins which carry oxygenated blood from the lungs. Arteries carry oxygenated blood away from the heart. Capillaries facilitate the exchange of nutrients and gases between the blood and tissues but do not specifically carry blood back to the heart. Lymphatic vessels are part of the lymphatic system responsible for immune function and do not directly carry blood back to the heart. Therefore, the correct answer is veins.

Correct Answer: B

Rationale: The quadriceps, a group of muscles located on the front of the thigh, are primarily responsible for extending the knee joint. When the quadriceps contract, they work to straighten the knee, allowing for actions like kicking or standing up from a seated position. The hamstrings (Choice A) are responsible for flexing the knee, the calves (Choice C) primarily work on plantar flexion at the ankle joint, and the glutes (Choice D) are mainly involved in hip extension.

Correct Answer: A

Rationale: Insulin is the correct answer. Insulin is produced by the pancreas and plays a crucial role in regulating blood glucose levels. It facilitates the uptake of glucose into cells, helping to lower blood sugar levels. Glucagon, on the other hand, is also produced by the pancreas but functions to raise blood glucose levels by stimulating the release of glucose from the liver. Somatostatin is another hormone produced by the pancreas that helps regulate the endocrine system. Cortisol, produced by the adrenal glands, plays a role in stress response and metabolism, not by the pancreas.

Correct Answer: A

Rationale: The main function of the circulatory system is to transport oxygen and nutrients to the body's cells while removing waste products. Oxygen and nutrients are carried by the blood to various tissues and organs, ensuring they receive essential elements for their proper function. Waste products are then removed from the cells and transported to the appropriate organs for elimination. This process ensures the overall well-being and functionality of the body. Choice B, removing waste products, is partially correct as one of the functions of the circulatory system is indeed to eliminate waste from the body. However, the primary function is to transport oxygen and nutrients. Choice C, regulating body temperature, is incorrect as temperature regulation is mainly controlled by the body's respiratory and integumentary systems. Choice D, protecting organs, is not a primary function of the circulatory system; protection is more closely associated with the immune system and the skeletal system.