What is the characterization of a cell membrane that controls which substance can enter and exit the cell?

Aboy jumps into a cold swimming pool and his body tempature goes down hus muscles ,blood vessels

You are working in the lab and are working with the same element but with varying isotopes of the element. you are working with c-11, c-13, c-14, and c-15. what is the average atomic mass of this element? (use periodic table) a- 12 b-15 c-14 d-13

To be or not to be that is da question