SAT English Practice Question

I have never been to the Arctic, but I have been to Antarctica. You could call the Arctic the "top" of the world and Antarctica the "bottom." The Arctic is a frozen ocean surrounded by land, while Antarctica is a continent of land surrounded by ocean. Although the two regions are very different, they are similar in some ways. In both regions, the temperatures are extremely cold, the winters are long and dark, and the landscapes are vast and breathtakingly beautiful. Both regions are also home to unique ecosystems and wildlife. What is the function of the underlined sentence in the passage?

1. A.offer evidence to support a previous claim.
2. B.introduce a comparison between two related ideas.
3. C.describe the similarities between the Arctic and Antarctica.
4. D.explain why the two regions are so different.