Hey Sandi,
Good question - the GMAT is pretty good about mixing up its answer choices so that some of those old high school strategies ("always guess C!") don't work anymore. But with Data Sufficiency, there's actually a little bit of "strategery" to guesswork, given that the reward system for those questions looks a little like:
D (give me whatever information you have and I'll solve it)
A or B (I can do it with just one statement)
C (I need two statements, but I can still do it)
E (I just can't do it)
When you look at it that way, it makes more sense for the GMAT to reward those examinees who can do more with less information, so you'll probably find that E is only the correct choice maybe 12-15% of the time, but not the full 20% that it would be on other questions. If you need to guess on DS, pick one of the "higher level" choices, because that's where the test is more likely to reward you.
Getting away from simply guesswork, you can use this hierarchy to avoid picking an "obvious" E or C - if those answers seem like gimmes, then you probably want to check the next answer above it to make sure, since they're unlikely to reward you for a lower-tiered choice unless you can prove that you're one important step away from the higher choice.
One other note about guesswork or just answer-letter selection - don't be afraid to pick A in Sentence Correction! Often times people feel as though the original must be flawed, but A is correct 20% of the time on those, so if you're guessing wrong you may want to see how often you're shunning a correct answer A, and train yourself to be more "tolerant" of that original choice. I've seen a lot of people make that mistake in the past...
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
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