Alveoli, tiny air sacs of the lungs which allow for rapid gaseous exchange.
I don’t quite understand what your asking but if your asking if it’s a mystery then no it isn’t but if this was not the answer you were looking for I suggest you put it another way that makes more sense
The independent variable is the water temperature because it is not dependent on the change of another variable so the answer would be D
Population Size, Good Luck!
It's called a nutrient.
Nutrients are well, something that all humans need to survive. A certain amount of nutrients keep humans alive and healthy, yet a lack of nutrients will cause them to become sick and slowly die.
Thus, making it so nutrients is a chemical substance that organisms require to live.
Hope this helps!