Which one of the following statements is true of fishes that live in freshwater?
a. The environment is isosmotic with respect to their tissues. As a result, they do not require a specialized organ to maintain water and electrolyte balance.
b. They lose water to their environment primarily through the gills. They replace this water by drinking.
c. Water enters epithelial cells in their gills via osmosis. Electrolytes leave the same cells via diffusion.
d. They have specialized cells that actively pump $\mathrm{Na}^{+}$ and $\mathrm{Cl}^{-}$ from blood into epithelial cells, so the ions can be excreted.