I don't understand how C is stated in the question stem, to me it seems that both C and D are necessary assumptions to the argument. I can see that D is arguably more related to the root issue, and perhaps therein lies the trickery of the test writers. Sill though, I feel strongly that both are necessary assumptions. Please disabuse me of my ignorance!

This one bothers me cause the first statement is that "One year-olds ordinarily prefer the taste of sweet food to that of salty food" and I felt like that fact especially because of the word ordinarily established that a preference for sweet food already existed and was therefore not an assumption but an objective truth for the purposes of the question.

please disabuse me of my incompetence haha

I fundamentally misunderstood the question. I pretty much forgot the application when I went to the answers. The application is that she should NOT buy the vase. C perfectly supports her buying it. but the should NOT buy it... anyways obviously E is perfect I just feel dumb now.

Im super annoyed. I thought I mapped 5 correctly but he still mapped it in revers of what I did.

I screwed it up cause I used "must" as a necessary condition indicator, and reversed the last statement. I know they always say these indicator words are under and over inclusive, but how should I know when that will be the case?