Describe the four major spheres that support life on Earth. Explain how rays AB and AC form both a line and an angle.

Everything in Earth's system can be placed into one of four major subsystems: land, water, living things, or air. These four subsystems are called "spheres." Specifically, they are the "lithosphere" (land), "hydrosphere" (water), "biosphere" (living things), and "atmosphere" (air).

Two rays AB and AC having the same endpoint are opposite rays if they two rays are unequal but line AB is equal to line AC. Otherwise they are nonopposite. An angle is the union to two nonopposite rays AB and AC sharing the same endpoint. The angle is denoted by either angle BAC or angle CAB.