Aresearcher places dogs in a cage with metal bars on the floor. the dogs are randomly given electric shocks and can do nothing to prevent them or stop them. later, the same dogs are placed in a cage where they can escape the shocks by jumping over a low hurdle. when the shocks are given, the dogs do not even try to escape. they just sit and cower. this is an example of

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands