Questions: The defining characteristic of a mammal is: (A) they have lungs (B) they have mammary glands for milk production (C) they lay eggs (D) they are endothermic (E) they have a four-chambered heart

The answer is (B): they have mammary glands for milk production.

Explanation for each option:

(A) They have lungs: This is incorrect. While mammals do have lungs, this characteristic is not unique to mammals. Many other animals, such as birds and reptiles, also have lungs.

(B) They have mammary glands for milk production: This is correct. The presence of mammary glands, which produce milk to feed their young, is a defining characteristic of mammals.

(C) They lay eggs: This is incorrect. While some mammals, like monotremes (e.g., the platypus and echidna), do lay eggs, the majority of mammals give birth to live young. Egg-laying is not a defining characteristic of mammals.

(D) They are endothermic: This is incorrect. Although mammals are endothermic (warm-blooded), this trait is shared with birds and is not exclusive to mammals.

(E) They have a four-chambered heart: This is incorrect. While mammals do have a four-chambered heart, this characteristic is also found in birds and crocodilians, so it is not unique to mammals.

In summary, the defining characteristic of a mammal is the presence of mammary glands for milk production.