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Choice A is the best answer because it most accurately describes how the underlined sentence functions in the text as a whole. The first sentence explains that Bernabei and his team studied growth rings to obtain information about the ancient oak planks found during a construction project in Rome. The next sentence presents what the researchers learned: the wood from the planks came from France’s Jura region, which is far from Rome. The underlined sentence then presents the implications of the findings about the planks: the wood must have been brought to Rome by boat, a difficult task that suggests Roman trade routes were complex. Thus, the underlined sentence mainly functions to present a conclusion about Roman trade routes based on the team’s findings.

Choice B is incorrect because the text doesn’t suggest that the team thought the ancient planks were used in the construction of a boat, nor does the underlined sentence question that conclusion. Instead, the text states that the wood could only have been transported from Jura to Rome in a boat. Choice C is incorrect because the underlined sentence simply offers a conclusion drawn from the team’s findings about the likely place of origin of the ancient planks; the text never mentions why oak was chosen for the planks instead of other wood. Choice D is incorrect because neither the underlined sentence nor the text as a whole addresses any methods that Romans used in constructing subways. Instead, the underlined sentence offers a conclusion drawn from the team’s findings about the likely place of origin of the ancient wooden planks discovered.