C. because stimuli is how your body responds to something, and in this case, someone's heart rate increases WHEN their blood oxygen drop. Do you see how it responds? ;-)
What is the question or paper? there isn’t really a question here if there is nothing to look at lol
Scientists rely on embryological development patterns and adult body plans for all animals in order to classify animals. The correct option among all the options that are given in the question is the third option or option "c". It is the work of the biologists to classify animals and put them in order for studying them easily and in a group.