Hi VJesus12,

We're told that K^2 = M^2. We're asked which of the following MUST be true. This essentially means "which of the following is ALWAYS true no matter how many different examples we can come up with?" These types of questions often require a bit more 'thoroughness' than normal - so you might have to do a bit more work than you initially think you will have to.

When dealing with squared terms, you should be prepared to look for multiple answers (for example X^2 = 4 has TWO solutions: +2 and -2). Here, we have a squared term equal to ANOTHER squared term, so there will be FOUR possible outcomes. Here's how you can TEST VALUES to quickly 'map' those options out:

K=3, M=3
K=3, M= -3
K= -3, M=3
K= -3, M= -3

Thus, we could have two positives, two negatives or one of each. Using these 4 options, you can quickly run through the answer choices and eliminate any answer that does NOT account for all 4 options. For example, Answer A can be eliminated because K=3, M=-3 is not a solution to that equation. You'll find that only one answer covers all 4 options...

Final Answer: E

GMAT assassins aren't born, they're made,
Rich