Answer:
Protozoa are single-celled eukaryotes
Explanation:
Eukaroytes are organisms whose cells have nuclei
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!
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Well first i just wanna say that life on mars has been discovered,not little green men but Bactria found on the surface and small traces of water have been found as well. some forms of life need little water to survive (like bacteria) but they still need water so yes there is water on mars the question is will we find it.
