How are living mammals classified into groups?

a. type of circulatory system

b. method of development

c. method of respiration

d. method of regulating body temperature

Mammals are type that feeds milk to its young and that usually has hair or fur covering most of its skin. Living mammals classified into groups through method of development. Mammals are warm-blooded higher vertebrates. E. g. Marsupials, monotremes and humans.