In which part of the ocean is mixing of water most vigorous? from 2 to 3 kilometers down from 1 to 2 kilometers down very deepest portions surface?

The part of the ocean that is mixing of water most vigorous is from 2 to 3 kilometers down. Thermocline is a layer between warm water from the ocean’s surface and cool water from below the ocean. In here, the temperature decreases rapidly from the warmer layer to the colder layer.  A thermocline forms due to the heat of the sun heating the ocean’s surface. Because of the difference in density between warm and cooler ocean water, cooler ocean water sinks and warmer ocean water floats. This is caused due to the heat and mass transfer between particles of the ocean. The answer is letter C. The sun’s radiation does not extend below a certain depth; therefore, deeper ocean water is colder than surface water.

Hmmm i'm not sure about this one