When it comes to energy, saying that something is cleaner than coal means that it does not harm the environment as much as burning coal does and it is usually sustainable. The clean is the same one in the term "clean energy"
The correct answer is the crust.
The crust is present above the mantle and is the hard outer shell of the Earth. The crust is 0 to 32 km in thickness. The densest type of crust is oceanic crust with the density of 3.0 g/cm3. The mantle is the layer below the crust and above the core.
The mantle exhibits an average density of 4.5 g/cm3. The density increases with depth as the pressure increases. The outer core exhibits a density between the range of 10 g/cm3 to 12.3 g per cm3 and the density of the inner core is about 12 g/cm3. Showing that the inner core exhibits the highest pressure.
