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Since we're looking for the answer choice that must always be true,
we need to find numbers that will make the answer choices false,
so that we can eliminate choices:
Let's pick numbers. Remember to pick numbers that follow the rules set
out in the stem, but try to find numbers that will make the answer choice
false.
Choice (A): If we pick x = 10 and y = –10,
and put them into Choice (A) we get  .
But the answer choice says that it should have been greater than or equal
to 5. Since 0 is NOT greater than or equal to 5, this choice is false,
and we eliminate it.
Choice (B):  .
Since 10 plus 4 equals 14 which is NOT less than 13, we eliminate this
choice too.
Choice (C):  .
None of our number selections are making this false, so let's keep it
for now and check out the other choices.
Choice (D):  .
Eliminate.
Choice (E):  .
Eliminate.
Once we've shown that all of the other choices can be false, we know
that Choice (C) must be correct.
Could we have stopped evaluating when we reached Choice (C) or did we
need to test all the choices?
Because this is a "must be true" question, we had to test them all.
That makes sense when you think about it, you can prove that one of the
choices is false by finding numbers that follow the rules set out in the
stem but make the answer choice false. However picking numbers couldn't
prove that Statement (C) was true, after all, it could simply have
been that we hadn't thought of the right numbers that would have made
it false! So, we had to eliminate all 4 of the others before we could
be certain that Choice (C) was the correct answer.
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