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Thanks MartyMurray

I rejected option A on the basis of reasoning that there can be other checks as well later since the given info only talks about initial checks "the Luhn algorithm dominates initial credit-card checks". Is my reasoning correct?
I tend to agree with your reasoning, though I feel more comfortable eliminating (A) by using the fact that the passage mentions only two things that the algorithm checks for, meaning that there could be other issues that it does not catch.
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