TOEFL Reading: ETS-TOEFL阅读机经 - 62CNNIWBRW98BX4FL$

Why does the author discuss "fishes such as skipjack tuna?" A. To explain by contrast why the circulatory anatomy of porpoises is efficient B. To show that marine animals other than mammals use a countercurrent exchange system to minimize heat loss C. To identify and illustrate a type of circulatory anatomy that is common in fishes D. To provide evidence that the amount of heat marine animals lose increases with increased body surface area