The specialized structures of complex animals have evolved because
A) the environment imposes identical problems regardless of where the animals are found.
B) the development of the specialized structures in an animal is influenced by the animal's ability to learn.
C) the simplest animals are those with the most recent appearance among the biota.
D) they permit adjustments to a wide range of environmental changes.
E) the most complex animals are the ones with the most ancient evolutionary origin.
Answer: D