Of All of the Chordate Groups, Why Are There Only

Of all of the chordate groups, why are there only two amniote groups (reptilia and mammalia) ?

A) Amniotes are the only chordate group that has successfully survived the transition to land.
B) The other craniate groups had brains that were not developed enough to allow the organisms to develop tool-building survival skills.
C) The vertebral columns of the other vertebrates were not strong enough to survive the transition to land.
D) The other jawed vertebrates were not able to obtain enough food to survive the transition to land.