Tissues of Sharks Are Isotonic to Seawater, but Their Concentrations

Tissues of sharks are isotonic to seawater, but their concentrations of sodium ions, potassium ions, and chloride ions in cells and extracellular fluids are similar to those of freshwater fishes. What can you infer about the movement of sodium and chloride in these animals?

A) To maintain homeostasis of sodium and chloride levels, the shark must take up additional sodium and chloride from seawater.
B) Sharks conserve sodium and chloride, limiting excretion.
C) Sodium and chloride must be eliminated through the gills.
D) Sodium and chloride will diffuse through shark gills from seawater down their concentration gradient.