Which statement correctly defines fish?

Which statement correctly defines fish?

• A. Mammals that live in water and breathe with lungs.
• B. Aquatic vertebrates that breathe with gills and are cold-blooded.
• C. Invertebrates that do not have backbones.
• D. Reptiles that live in water and have scales.

Answer: (B)

Fish are aquatic vertebrates with gills and are cold-blooded. They live in water, have a backbone, and exchange gases through specialized gill membranes that extract dissolved oxygen from the water. Their body temperature generally matches the surrounding environment, rather than being actively maintained internally.

Other descriptions miss one or more essential traits: mammals that live in water breathe with lungs, not gills, and regulate their body temperature; invertebrates lack backbones; and aquatic reptiles live in water but do not have the same combination of gills and cold-blooded metabolism. The defining mix—living in water, having a backbone, breathing through gills, and being cold-blooded—best describes fish.