These four subsystems are called "spheres." Specifically, they are the "lithosphere" (land), "hydrosphere" (water), "biosphere" (living things), and "atmosphere" (air). Each of these four spheres can be further divided into sub-spheres
The answer you're looking for is C<span />
Sand sheets and dunes cover approximately 25 percent of the Sahara's surface.
Our planet, Earth appears blue due to the mass amounts of water and lacks on its surface.
