The operative words are "main point". Tip: I think it is always a good idea to start with this kind of question. It will allow you to determine what GMAT thinks is the main idea.
Compare And Contrast The Answer Choices:
(B) Reject. Although this is true it is far too narrow to qualify as the main idea.
(C) Reject. There is no discussion in the passage about previous perceptions and the necessity to revise them.
(D) Reject. If anything this is the opposite of what you would expect. The passage seems to assume that the laboratory studies ARE indicative of the physiological behavior of seals when the dives last more than 20 minutes.
(E) Reject. This is too narrow. Although true, it does not qualify as being the main point.
(A) This is the best answer. It is neither overly broad not overly narrow and is consistent with what the passage is about.
Back to the tutorial. Go to question 5.
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