Answer
b.
Work Step by Step
About 3.5 billion years ago, prokaryotes were the only living things inhabiting Earth. It remained this way until about 2 billion years ago when they were joined by other life forms.
Options a., c., and d. are incorrect because these options depict a world where no life existed yet.
Therefore, option b. is correct. If we were to go back to 3 billion years ago, we would have seen prokaryotes, which would have formed the green scum in the water.
