Easy. Answer is oxygen because it says which one was missing before any life was on earth. And in order to have life on earth you need oxygen!
:)
Answer:
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tissues
Answer:
Contain a repair mechanism inherent in their biochemistry (repair enzymes)
Explanation:
When cells are exposed to sublethal doses of ionizing radiation, repair and recovery may occur because cells: Contain a repair mechanism inherent in their biochemistry (repair enzymes).