Two characteristics used to define mammals include
a. mammary glands and scales.
b. mammary glands and hair or fur.
c. mammary glands and sweat glands.
d. sweat glands and a four-chamber heart.

b. mammary glands and hair or fur. things also to classify: feed milk to young, give birth live, and breathe in air, so they have lungs.

b. they fed their child with milk from mammary gland and their body are covered up with hair or fur not feather