This Official Guide from GMAT outshines all others, simply because the questions are chosen from actual past tests and are very thoroughly researched. Equally important the answer choices for each question are also well thought out and both the correct and the incorrect choices reflect actual test patterns. The explanations for the answers in the Official Guide are crystal clear and very insightful.
The rules of grammar on which the GMAT Sentence Correction questions are based are best outlined in this guide. The section on Arguments is simply outstanding. I had a lot of difficulty with the Verbal section until I read through the explanations here.
The absolute best part of the guide is that it not only explains clearly why an answer choice is correct, but also explains why the other choices are wrong. This is most useful since it shows you the patterns and tricks used on the actual test to lead you astray from the correct answer. Once you see the patterns, you can quickly identify and rule out the incorrect answers and pick out the best choice.
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