I was just thinking about how recaptcha is getting harder when I thought about another posible solution. Images won't last forever so we will need something else some day - like human logic or emotion. Google and others are trying grouping images by category (find the image that doesn't belong) but that requires a large amount of images and doesn't work for the blind.
Anyway, what if a massive collection of text was gathered (public-domain books from each language) and a sentence was shown to the user with 1 (or 2) words that were a select box of choices? Only computers that knew correct English/Spanish/German grammar would be able to tell which of the words belonged in the sentence.
Would there be any problems with this approach? I would assume that it would be easy enough for anyone that knew the language that the sentense was displayed in to figure out the answer easier than trying to read the reCAPTCHA text. Plus, storing an insane number of sentences would only take a couple gigabytes of space and wouldn't take anywhere near the CPU time creating images/audio takes. In other words, anyone could host their own captcha system with minimal impact on system performance.
Is there a problem with this approach? More specifically I'm looking for the main problem with this approach.