The best and most correct answer among the choices provided by your question is the second choice.
The best comparison between birds and fishes is that: t<span>hey are both vertebrates, but fish are ectothermic and birds are endothermic. </span>
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They are both vertebrates, but fish are ectothermic and birds are endothermic.
Explanation:
Vertebrates are the animals which have a back bone or spinal cord. Fish, amphibians, reptiles, birds and mammals. Fish are characterized by streamlined body which helps them in easy swimming across heavy water flow and birds walk only in two legs so they are bipedal so 2nd option is the correct answer.
Between these two, only fish, amphibians and reptiles are ectothermic i.e. they cannot maintain their internal body temperature while birds and mammals are endothermic. So, 1st option is incorrect. Some fish are jawless but all of them are not jawless so 3rd option is also incorrect. Fish have two chambered heart but birds don't. Birds have four chambered heart so 4th option is wrong.
