The Key Difference Between Ectotherms (Such as Lizards)and Endotherms (Birds

The key difference between ectotherms (such as lizards) and endotherms (birds and mammals) is that

A) ectotherms get the energy they need for thermoregulation from the environment while endotherms generate heat metabolically from within their own bodies.
B) ectotherms can't regulate their body temperatures at all while endotherms can.
C) ectotherms get the energy they need for thermoregulation from their own metabolism while endotherms must rely on sunlight.
D) endotherms can't regulate their body temperatures at all while ectotherms can.
E) ectotherms have brown fat and endotherms do not.