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a layer of ozone gas which occurs naturally in the upper atmosphere, and protects life by absorbing harmful ultraviolet radiation.
the actions of the body's immune system that work to fight infections or disease.
a scientist who studies fossils.
an infectious disease caused by the bacterium Mycobacterium tuberculosis thatusually...
to change over a period of time in response to new environmental conditions.
(also called heterotrophic organisms) organisms that are not able to produce their own organic...