Relationships Among the Animals Having Body Cavities Can Be Easily

Relationships among the animals having body cavities can be easily determined by examination of embryonic development; vertebrates and echinoderms are grouped together because in each

A) the infolding that forms the body cavity eventually becomes the anus.
B) life starts as a single fertilized cell and then becomes multicellular.
C) the death of specific cells creates the internal hollow that eventually becomes the body cavity.
D) the infolding that forms the body cavity eventually becomes the mouth.