Which is the correct description of a thermocline?

a.
an area of the ocean that separates water based on the mass of its dissolved minerals

b.
an area of the ocean that separates the warm surface waters from the colder deep waters

c.
a horizontal area that separates the top layer of high nutrient ocean water from the low nutrient ocean water below

d.
a vertical area of density change that occurs when temperature and salinity change