A．They're warm-blooded．
B．They have a set of teeth of different shapes．
C．The first period of development of young mammals is within the mother's body．
D．There's some association between mother and infant．
A．Mammals developed from the reptiles．
B．The animals that are hatched from eggs have no opportunity for learning．
C．Mammals developed at the cost of the extinction of reptiles．
D．Not all the primates are mammals．
A．from a special kind of dinosaurs
B．from peculiar mammals
C．after the extinction of dinosaurs and other reptiles
D．after the extinction of all small mammals
A．they are hand-feeders
B．of their anatomical features
C．they are arboreal
D．they depend on each other