With literary texts, there may well be certain limitations because there will often be creative use of structures. However, I don't see that this would necessarily be a problem- it depends on the choice.
I've just downloaded the Kurt Vonnegut. Most of the examples seem fairly clear for article usage. There will be cases with articles where there is more than one possibility for an answer- for instance, in the third paragaph, line 6, the text has 'from the crewmen and planes', but it would be possible to have 'and the planes'.
On a presentational point, when I choose the wrong answer, the text automatically adds the correct choice. Would it be possible to signal to the user that they have made an error?