NOTE: This question is not representative of an actual GMAT question, but the strategy used to determine the answer is applicable on some GMAT problems.
Q. Which of the following must be true?
x > 1
x > 2
x > 3
x > 4
x > 5
How can this question be answered?
Though this is an extreme example, logic will sometimes eliminate answer choices since only one answer can be correct and one answer has to be correct.
If x is greater than two, then it must also be greater than one. Both answers can't be correct so answer B cannot be correct. The same thing is true of each other answer too--if x is larger than 3, 4, or 5 it is larger than one. C, D, and E cannot be selected without multiple answers being correct. The only answer that doesn't force any other answer also be true is A.
This doesn't mean that a less restrictive answer is always correct, simply that a more restrictive answer cannot be correct.
Additionally, on Roman numeral problems, if one Roman numeral statement is more restrictive, and inclusive of, than another, confirmation of the more restrictive one confirms the other. Conversely, disqualification of the less restrictive statement also disqualifies the more restrictive statement.