[shiro]

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.