Scientists Hypothesize That Multicellularity Evolved When Groups of Cells Exposed

Scientists hypothesize that multicellularity evolved when groups of cells exposed to an internal environment specialized to perform tasks needed by groups of cells exposed to the external environment, and vice versa.Which biological structures existing today could be used as examples to explain this concept?

A) The blood cells moving in a fish's circulatory system and the fish's lung tissue
B) The lining of a snake's intestine and the snake's skin
C) The bark on an oak tree and the waxy cuticle on an oak leaf
D) The trachea (airway) in a blackbird's neck and the heart in the blackbird's chest
E) The whiskers on a cat's face and the fur on the cat's tail