The body cells of most multicellular animals are not in direct contact with the external environment. These cells are supplied with materials from the environment by:
A. sense receptors.
B. hydrolytic enzymes.
C. a circulatory system.
D. a central nervous system.
A circulatory system contains three components: a fluid to transport materials, a pump to pump the fluid to all cells, and a network of vessels to carry the fluid. A central nervous system will control and coordinate the body, hydrolytic enzymes will break down complex food molecules in the digestive tract and sense receptors will detect stimuli.