Final answer:
The sphere composed of all living organisms is the biosphere. The other spheres, the hydrosphere and the lithosphere, represent the water and solid parts of the Earth respectively, and while they do support and interact with life, they are not composed solely of living organisms.
Step-by-step explanation:
The sphere that is composed of all living organisms is called the biosphere. The biosphere encompasses all parts of the earth where life exists, from the deepest parts of the ocean to high up in the atmosphere. The other spheres mentioned, the hydrosphere and the lithosphere, refer to the water component and the solid earth component of our planet respectively. While they do support and interact with life, they encompass much more than just living organisms. For example, a rock in the lithosphere is not a living organism, nor is an isolated pool of water in the hydrosphere.
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