We can see straight away that there is a link between sentences C and E. Sentence C introduces cyanobacteria, and sentence E tells us how these microbes lead to the increase of oxygen in the atmosphere. Now, sentence B too talks of the levels of oxygen in the atmosphere, describing how some of it was fixed by iron and organic matter on the earth, while the rest was just added to the oxygen in the atmosphere. CEB is a unit. Sentence A talks of “this” leading to the “burning off” of existing life forms. What led to this? The word ‘poisonous’ in sentence B gives us the clue. The increase in the level of oxygen, which was poisonous to the life forms that existed 2.4 billion years ago, lead to their burning off. So, we have the arrangement CEBA. Answer options (b) and (d) differ only in the placement of sentence D- should it conclude the paragraph or should it start it off? Sentence D talks about a massive upheaval of the earth's atmosphere. If it were to follow sentence A, then A too should talk about changes in the atmosphere, especially as sentence D refers to "one such massive upheaval". But sentence A talks of burning off of life forms that existed on Earth at the time and of the 500 million years it took for oxygen-using forms to appear. So D does not make sense after A. On the other hand, if D were to start the paragraph, we see that the rest of the sentences explain the "Oxygen Catastrophe”. So DCEBA is the correct order. Correct Answer: DCEBA
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