Most Animals Digest Dietary Proteins into Their Constituent Amino Acids

Most animals digest dietary proteins into their constituent amino acids and then synthesize new proteins because

A) all amino acids are essential and cannot be synthesized by animals.
B) though protein function is often species-specific, protein structure is not.
C) foreign proteins are considered invaders and are attacked by the immune system.
D) animals build proteins by first synthesizing acetyl groups from the breakdown products of protein.
E) though macromolecules such as proteins can pass freely into the intestinal cells, they cannot then be transported into the blood vessels.