GMAT Critical Reasoning (CR) | Re: CR Revision: Studies in restaurants show that the tips left by

Hey,
I think there are some gaps in my understanding and I would really appreciate if someone can clarify them. What is wrong with option A? I agree it strengthens the argument but it does so lesser than C. Is my understanding correct? Can there more than 1 strengthener?
I feel so it removes an additional parameter, the mode of payment, thus solidifying the "cause and effect" relation indicated in the argument.

Please let me know. Don't wanna chose the wrong option on my GMAT
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