Why Are Polar Oceans Most Fertile in the Autumn

Why are polar oceans most fertile in the autumn?

A) The warm summer temperatures allowed for the growth of many microorganisms in the water.
B) As the oxygen rich surface waters cool in the autumn, they sink to the bottom, displacing the layer of decomposing organic matter, forcing it to rise, adding nutrients to the surface of the polar oceans.
C) The salinity of the ocean is at a low point at that time of year.
D) Ocean currents tend to reverse during the autumn, bringing nutrient rich waters up from the equatorial regions.