What is a thermocline?

Explanation:

a thermocline is an abrupt temperature gradient in a body of water such as a lake, marked by a layer above and below which the water is at different temperatures.

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i think it would be b. tissue because an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.

answer; polycythemia is the correct answer;