TOEFL Reading: ETS-TOEFL阅读机经 - 33H50NMBZ2HY3218L$

According to paragraph 4, what is a "thermocline" A. The deep, cold layer of water below the surface of oceans and lakes B. The thin layer of warm water on the surface of oceans and lakes C. The change in density of surface water as it is continually heated D. The point at which the temperature of water in oceans and lakes changes from warm to cold