The animal kingdom is made up of invertebrates (animals without bones) and vertebrates (animals with bones),
Vertebrates are divided into five classes, or subphyla.
These five classes are mammals, birds, fish, reptiles, aná
amphibians, Humans are classified as mammals, or warm-
blooded vertebrates that nourish their young with milk.
Which of the following descriptions explains what mammals are?
A. The five classes of vertebrates
B. Warm-blooded vertebrates that nourish their young with milk
C. Animals without bones
D. Invertebrates and vertebrates