Explanation
Correct answer: D.
Choice D is the best answer. “By contrast” logically signals that the finding described in this sentence contrasts with the finding described in the previous sentence. That is, quicker blinks were associated with longer talking times, whereas slower blinks were associated with shorter talking times.
Why the other choices are wrong
Choice A
Choice A is incorrect because “for example” illogically signals that the finding described in this sentence is an example of the finding described in the previous sentence. Instead, it contrasts with that finding.
Choice B
Choice B is incorrect because “specifically” illogically signals that this sentence provides specific, precise details about the finding described in the previous sentence. Instead, it presents information that contrasts with that finding.
Choice C
Choice C is incorrect because “firstly” illogically signals that the finding described in this sentence is a first point or occurs first in a chronological sequence of events. Instead, it contrasts with the finding described in the previous sentence.