Bats are mammals and, by definition, females possess mammary glands which produce milk to nourish their young. Also, birds do not have mammary glands and feed their young in other ways (e.g., with chewed food or regurgitated food).

A. possess four chambered heart: Both bats (mammals) and birds have a four-chambered heart, so this characteristic does not differentiate them.

B. are active at night: While many bat species are nocturnal (active at night), this is a behavioral trait, not a defining physical characteristic that differentiates all bats from all birds, as some bird species are also nocturnal.

C. are warm blooded: Both bats and birds are warm-blooded (endothermic), meaning they can regulate their own body temperature, so this characteristic is shared, not a differentiator.