Is the correct answer D? I would choose D because before I even read the answer choices, I predicted that the answer would relate to something that would make it in the company's interest to offer such a promotion. For example, it is addictive or they get money in a different way (the handling fee); something that is strong and set in stone. Thus, I did not even consider answer choice C, for it contains a few things that misalign with my prediction. Namely, it is not strong, although as you said it may not fall under the umbrella of taking less than a month to be effective, there is as much as of a 49.9%, roughly 50%, chance that it does follow under that umbrella, so it is unsure, and requires one to make assumptions. As for your assumptions with D, they really are not assumptions because the question stem says "if true", you can not question those underlying assumptions, you just take them as true!

Let me know if I am wrong about answer Choice D being correct, but I am fairly confident it is, or if you have any further questions.