Nhận biết (M1)

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 __________

Which choice most logically completes the text?

• A.

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

• B.

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

• C.

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

• D.

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