Salmon Eggs Hatch in Freshwater

Salmon eggs hatch in freshwater.The fish then migrate to the ocean (a hypertonic solution) and,after several years of feeding and growing,return to freshwater to breed.How can these organisms make the transition from freshwater to ocean water and back to freshwater?

A) The rectal gland functions in the ocean water,and chloride cells function in freshwater.
B) Different gill cells are involved in osmoregulation in freshwater than in salt water.
C) Salmon surface to breathe oxygen,so there is no need for more than one mechanism of osmoregulation.
D) Their metabolism changes in salt water to degrade electrolytes.