Explanation
Correct answer: B.
Choice B is the best answer. “Thus” logically signals that the information in this sentence—that the researchers concluded that Antarctica had glaciers long before the formation of its continent-wide ice sheet—is a result of the previous information about the discovery that Antarctica’s glaciers are at least 60 million years old, whereas its continent-wide ice sheet formed just 34 million years ago.
Why the other choices are wrong
Choice A
Choice A is incorrect because “by contrast” illogically signals that the researchers’ conclusion about Antarctica having glaciers before its ice sheet formed contrasts with the previous finding about the age of Antarctica’s glaciers. Instead, the conclusion is a result of that finding.
Choice C
Choice C is incorrect because “nevertheless” illogically signals that the researchers’ conclusion about Antarctica having glaciers before its ice sheet formed is true despite the previous finding about the age of Antarctica’s glaciers. Instead, the conclusion is a result of that finding.
Choice D
Choice D is incorrect because “even so” illogically signals that the researchers’ conclusion about Antarctica having glaciers before its ice sheet formed is true despite the previous finding about the age of Antarctica’s glaciers. Instead, the conclusion is a result of that finding.