GMAT Sentence Correction is often very similar to Algebra in terms of the way sentence structure is tested. The usage of EITHER-OR best exemplifies this.
On the GMAT it will always boil down to a question or two here and there. As we have discussed in an earlier post, you can make only about 11-12 mistakes in the Verbal Section to score reach a 700, provided you get a 49-50 on Quant, making about 7-8 mistakes or fewer. While it might seem like a healthy margin for error, when we break it down into question-types, the margin for error will not seem that wide.