Great question - thanks for posting this one (and for the invitation to chime in!).
This is a terrific example of two crucial strategies for Assumption questions:
1) You have to identify the exact conclusion of any CR strengthen/weaken/assumption question
2) The assumption is something that is necessary
to the argument; not merely something that would strengthen it
Here, the conclusion is that "prohibiting the sale of video games to minors would help curb
carpal tunnel syndrome among adolescents. That's entirely different than something that would eliminate it.
The problem with choice C as it pertains to this conclusion is that the conclusion doesn't require video games to be the only or even a primary reason that kids develop this problem. Even if only 5% of carpal tunnel cases were related to video games, eliminating video games would still "help curb" the problem, however slightly. We don't need for video games to be the only reason that kids get this problem...we just need it to be one reason.
Now, with D, we need that to be the case - prohibiting "the sale of games to minors" doesn't necessarily mean that minors won't still play the games. In order to curb the problem we need to stop kids from playing the games, not simply from buying the games. If D were untrue - if it were the opposite - then we could say that parents will just buy these games for their kids anyway, so the kids won't stop playing the games. Therefore, we need D to be true - it's required by the argument otherwise the conclusion (that this law will help reduce the problem) isn't supported at all.
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