I'm new to Cassandra, but querying in it is relatively limited, compared to, for instance, a relational database. (This is by design.) I'm pretty sure there's no support for full text search at this time (this may not even be on the roadmap).
You might be best to go with Lucene or something comparable to index the text of the questions, either within the Cassandra datastore or in a separate datastore.
There appears to be at least one project that is attempting to integrate Lucene with Cassandra, and there may be others:
Another way to go in your case might be to break up the questions into words and maintain your own index of words to questions; your mileage may vary here, and something like Lucene will no doubt give you greater flexibility in querying.