Which best defines the biosphere?

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Life thrives where the land, air, and water come together and influence one another, encompassing all living organisms and the environments that support them. This is what the biosphere captures: the global system of life and the worldly contexts that sustain it, from soils and rivers to the oceans and the air we breathe. The best definition here emphasizes this interconnected, life-supporting zone rather than a single medium. The other options name individual parts of Earth—the atmosphere, the hydrosphere, or the lithosphere—but none alone describes the system that enables life through the interaction of land, air, and water. A forest, a lake, or a coral reef illustrates the biosphere in action, where organisms depend on and shape their surroundings across these media.