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? ;-)
Oxygen.
Oxygen was fatal when organisms started using photosynthesis, this event was known as the Great Oxidation Event.
It is about 78%, I hope that this helped :)
Answer:
sorry I needed points ehjejejejrjrj4nn4j4j4
Your answer is going to be B
