Improving your overall Verbal pacing would obviously be the best thing to do. If you haven't already, you should experiment with different strategies, to find which gives you the best balance between speed and accuracy. People are different, and have different skills (read at different speeds, retain different amounts when they read), so a strategy good for one test taker might be bad for another. Note taking, for example, is a very good idea for some test takers, and a waste of time for others. So don't commit to any particular approach unless you find it gives you good results.
But to answer your question, if you knew with certainty you couldn't finish the Verbal section on time, and would need to guess instantly on one or two questions later in the test, you'd normally consider two factors:
- on which question type will I save the most time?
- how will a random guess affect my score?
If we're assuming you've already invested time in reading an RC passage, an RC question should take you less time than a CR question, and CR normally takes longer than SC. So you'd save the most time guessing at CR.
In terms of scoring, most of the time the question type won't make a difference, if you're guessing later in the test. But RC is less adaptive than CR or SC - a passage will have a general difficulty level, but individual questions might be easy or hard. So when you guess instantly at RC, you're using a higher-risk strategy. Sometimes you'll be guessing at an easy question, which will hurt you a lot. And sometimes you'll be guessing at a hard question, which won't hurt you much at all. A guess at CR or SC is more likely to be a guess at a question around your level. So you're gambling more by guessing instantly at RC, and an RC guess would make sense only if you're willing to accept the tradeoff - a higher chance of a low score, but a small chance of a better-than-expected score. Most well-prepared test takers should be trying to minimize their risk of a low score, so should not be guessing at RC. When you factor in the timing considerations, a guess at CR seems the best option for most people in this situation.
That said, random guessing at Verbal is usually a bad idea, if you're able to get down to two candidate answers within about a minute. If you can do that, it's slightly better, with 2 minutes and 2 questions left, to invest one minute in each question and guess 50-50 than it is to invest two minutes in one question, and randomly guess the other.
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