Salmon Eggs Hatch in Fresh Water

Salmon eggs hatch in fresh water. The fish then migrate to the ocean (a hyperosmotic solution) and, after several years of feeding and growing, return to fresh water to breed. Which statement best explains how these organisms make the transition from fresh water to ocean water and back to fresh water?

A) The rectal gland functions in the ocean water, and chloride cells function in fresh water.
B) The salt transport mechanisms of the gill epithelia change during migration.
C) Salmon in fresh water excrete concentrated urine, and salmon in salt water secrete dilute urine.
D) Their metabolism changes in salt water to degrade electrolytes.