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? ;-)
Answer:
matter
Explanation:
"In thermodynamics, a closed system can exchange energy (as heat or work) but not matter, with its surroundings"
Water is a key element and necessary thing life needs in order to live and thrive without water everything would die.
