A geothermal HVAC (heating, ventilating, and air conditioning) system air-conditions your house by passing warm air pulled from the house past a continuous flow of cool water pulled from the ground. How this heat exchanger works is analogous to:
a. countercurrent exchange of gases in fish gills.
b. diffusion of $\mathrm{O}_{2}$ from blood to cells in shark tissues.
c. diffusion of $\mathrm{CO}_{2}$ from cells to blood in crabs.
d. use of $\mathrm{O}_{2}$ in cells in insects.
e. excretion of $\mathrm{CO}_{2}$ from mammalian cells.