I think your answer would be b.
The largest desert on earth is the Sahara Desert in Northern Africa. The Sahara is 9,400,000 km2.
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