Hi Maggie,
Thanks so much for your comments and thanks for joining the site! — Look forward to helping you out in any way I can —
You are definitely right that an answer that would work as a sufficient assumption (that is, something that completely fixes the issue in the reasoning) would most certainly help strengthen the argument —
I meant to say as much in the second paragraph under “Solutions ” on page 288 (sufficient assumptions being a specific type of assumption) but I think that I was overly brief in my explanation (and it’s a bit hard to find), and I can definitely see how it could be difficult to relate that paragraph to the sufficient assumption solutions you found.
It’s great that you are developing such strong instincts about the exam (strong enough to question what you study, which is awesome) — I hope that clears it up, and if you need anything else just let me know — MK