AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
(A) Both A and R are true, and R is the correct explanation of A.
Mammals and birds have four-chambered hearts keeping oxygenated and deoxygenated blood separate, because they use energy to maintain constant body temperature and thus require efficient, uninterrupted oxygen supply.
The textbook (Ch. 5, section 5.4.1) directly states: "Such separation allows a highly efficient supply of oxygen to the body. This is useful in animals that have high energy needs, such as birds and mammals, which constantly use energy to maintain their body temperature." This makes R the correct explanation of A. Amphibians have three-chambered hearts and tolerate some mixing because they don't regulate body temperature. Both statements are true and R logically explains why A is true.