What CAPTCHA emphasizes is that it is automated. Not only the verification process, but also the process of generating problems. If the problem space is fixed, eventually the list of answers is formulated if it is worth. In practice if spammer's think it's not worth we can outperform them by more problems, but that's kind of passive, giving up control to them. In general, what matters is asymptotic ratio of our cost vs spammer's cost---if it is both linear, it's not very promising. If the ratio reaches zero asymptotically, it means the more we and spammers compete the better we do, hence the control is ours.
Great idea. And not a bad one that would be applicable to CAPTCHAS as a whole. If non-technical. E.g. ask the user like 'What color is a US Dollar Bill'
I've lived in the US, and I had to look at the dollar bills I happen to have in my wallet. Green, yes, but did they add other colors recently? Not a very foreigner-friendly question.
The problem with technical questions, is that no matter how simple, people here will argue about the correct answer ;-). Unless we just give up and say that "It depends" is always correct.