Which choice most logically completes the text?

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Carl Woese's 1970s study of methanogens (microbes with no nuclei, such as Methanococcus vannielii, that metabolize hydrogen and carbon dioxide in oxygen-poor environments and produce methane as a by-product) suggested that they did not fit scientists' then-conventional division of life into two domains, the eukaryotes (having cells with nuclei) and the prokaryotes (missing nuclei). Molecular clock evidence indicated that what Woese argued was a third domain of life emerged as early as 3.5 billion years ago. Methanogens would have been well-adapted to conditions then given that ______

atmospheric hydrogen levels were much lower around 3.5 billion years ago than they were both earlier and later in Earth's history.

methane, while primarily biological in origin on Earth, can be produced in the absence of biological activity.

the earliest undisputed evidence of eukaryotes is much more recent than 3.5 billion years ago.

Earth's oxygen levels were very low until the Great Oxidation Event approximately 2.4 billion years ago.