sat-dog

Choice D is the best answer. The first sentence claims that the number of sunspots can vary greatly. To support this claim, the next two sentences compare two examples: one time when there were 180 sunspots and one time when there were only 2 sunspots. So the transition “by comparison” fits perfectly.

Choice A is incorrect. This choice uses a transition that indicates a restatement of the same idea, which doesn’t make sense here. This sentence doesn’t restate the first example—it describes a totally different example. Choice B is incorrect. This choice uses a transition that indicates the addition of a similar idea, which doesn’t make sense here. This sentence describes a second example that is very different from the first example. Choice C is incorrect. This choice uses a cause-and-effect transition, which doesn’t make sense here. The first example didn’t result in the second example.