I think the answer is D. A good indicator is the resting heart rate. Exercise strengthen your heart and it makes it more efficient. The resting heart rate shows of the heart is pumping blood normally each time it contracts. Having a normal resting heart rate would mean a healthy heart.
I think the right answer is d!
Hello!
I believe the correct answer would be: B. Electric eel.
I really hope my answer helped you out! c: