The stimulus begins by introducing a phenomenon, that the waters off the Pacific coast of North America have warmed in recent years. In the second sentence, the author shares what some scientists believe about the warming. We know that sentence two is not the main point because it starts with “some scientists”. On Main Point questions, we are concerned with what the author thinks, not what critics, scientists, or anyone else thinks. Since the author took the time to share what the scientists believe, however, we expect the author to respond to the scientists. Perhaps the author will agree, but expect the author to disagree more often than not. Sure enough, in sentence three, the author responds by saying that the conclusion made by the scientists is far from justified. When the author refutes someone else’s argument, the statement saying that the other group is wrong, or not necessarily right, tends to be the main point.
The problem with answer choice B is that it represents what the scientists believe. We need an answer choice like C that suggests that what the scientists believe is not justified.