21. which 2 body systems work together to
produce and transport blood cells?

skeletal and circulatory

Explanation:

The skeletal system works with the muscular system to help the body move. Marrow, which is soft, fatty tissue that produces red blood cells, many white blood cells, and other immune system cells, is found inside bones.

The circulatory system consists of the heart and blood vessels which emcompasses all of the arteries, veins, and capillaries. The arteries carry oxygenated blood away from the heart, and veins return deoxygenated blood back to the heart. The main purpose of the circulatory system is to transport blood, oxygen, nutrients and hormones to and from different cells and tissues throughout the body. This system works hand-in-hand with the respiratory system to facilitate the exchange of oxygen and carbon dioxide within the blood per the alveoli in the lungs. It is also very important for the the removal of wastes and poisons within the body via the digestive and urinary systems.

i'm pretty sure it is letter c.) car