What's the color of a banana? How many words are in this sentence? Please type the first character of each word in this sentence!
The problem there is you have limited possibilities and you'd probably need to ship the list of questions with Dokuwiki. Of course there's always this http://www.hotcaptcha.com/ ;) -- DokuWiki mailing list - more info at http://wiki.splitbrain.org/wiki:mailinglist