Method for reading comprehension, explained in our classroom courses and in our on-the-shelf books, recommends that you always figure out the author's purpose as you're reading, before you even get to the relevant question. Figuring out the purpose is much easier as you go into the passage than as you re-read it for the third time, and knowing the answer in advance can help you rule out those tempting wrong answer choices. Meanwhile, having the author's primary purpose can also help narrow down answer choices for some of the trickier Inference questions.
Best of luck with your studying!
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