Answer:
The circulatory system, and respiratory
It means that the cell must only let in certain items and must only allow a certain amount of something into the cell
B is you're answer. I did this before
B) The enzyme has active sites where the substrate binds with the enzyme to form a complex. When the substrate binds to the active site, an induced fit is formed where the enzyme changes its shape in order to better serve the substrate and lower the activation energy of the reaction.