You said your "cs" stinks - did you mean sc or cr?
For CR (and RC), it's really important to study why the wrong answers are wrong. For any question (when studying), pick the wrong answer you think is most tempting and be able to articulate why it is so tempting, and why it is wrong anyway. Understand HOW they get you to pick the wrong answer and you'll be less likely to fall into that trap in the future.
For SC, it's all about the grammar. If you are falling into traps, you either don't know the rules well enough or you're relying on your ear too much, or some of both. Again, figure out why you're getting stuff wrong and then do what you need to do to fix that problem. If you're lacking on the rules, study the rules (both from the book you're using AND in the context of actual OG questions that test those rules). If you're relying on your ear too much, make yourself point to a word and articulate the specific rule being broken. If you can't do that, don't cross off that choice.
Generally, second-guessing yourself is a bad idea. Go with your original answer UNLESS you absolutely know you made a mistake with your first answer (and you could actually explain what that mistake was).
Think of it this way: when you pick an answer from among 5, you have a 20% chance that you're right and an 80% chance that you're wrong. If you're right and you change your answer, now you're going to get it wrong. If you're wrong and you change your answer, you have a 25% chance of switching to the right answer and a 75% chance of switching to another wrong answer. Unless you KNOW you made a mistake the first time around, the chances are pretty good that you don't really know what you're doing, so you're probably not going to get this one right. And, if you're probably going to get it wrong anyway, do you want to spend more time picking a new answer? Of course not - you want to get it wrong right now so you can have time for other problems that you might actually get right.
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Stacey Koprince
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Manhattan GMAT
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