Dissolved Oxygen Is Often Very Low at the Base of the Thermocline

Dissolved oxygen is often very low at the base of the thermocline in the open ocean because

A) Many fish congregate there
B) It is low from there, on down to the deep sea bottom, because of poor mixing with surface water
C) Fine-grained organic matter accumulates there, and microorganisms consume oxygen as they break down this material
D) This is the depth of the North Atlantic Deep Water, which is very low in dissolved oxygen