Extremophiles such as halophiles and extreme thermophiles are archeas which live in and can survive in extreme conditions. Halophiles thrive in high salt concentrations, while extreme thermophiles are used to temperatures like in hot springs (so, very high).
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? ;-)
