[ B. Earths atmosphere. ]
The atmosphere regulates the temperatures on Earth and protects it from harmful UV rays and meteors. In the stratosphere, it is heated by the ultraviolet radiation of the sun. If it weren't for the stratosphere, the rays from the sun would be deadly. In the troposphere, all of Earths weather occurs in it. The air in the troposphere is the densest as it contains 3/4 of all mass in the entire atmosphere.
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