Sponges, Cnidarians, and Flatworms Lack a Specialized Gas Exchange Surface

Sponges, cnidarians, and flatworms lack a specialized gas exchange surface because

A) they are too large for a circulatory system to operate well.
B) they live without need for oxygen.
C) they do not produce carbon dioxide.
D) countercurrent exchange mechanisms cannot function well in their living conditions.
E) nearly all of their cells are in direct contact with the external environment.