The Geothermal Gradient, About 25°C/km, Is Generally Valid for Which

The geothermal gradient, about 25°C/km, is generally valid for which of the following situations or uses?

A) to determine the depth at which the rock melts to form the outer core
B) to illustrate that variations in the temperature of the crust are minimal
C) to approximate near surface temperatures
D) to validate models showing increasing wave ray velocities with depth