Spammers getting through question and answer human verification
 

I had 100 spammers register yesterday. I use the Q and A human verification, and I can't figure out how they got through. The question doesn't seem (to me) to be solvable by a bot.

However, the spammers are getting through somehow, so I'm wondering if it is best practice to set up many different questions? Would that be more secure in some way? Are there any other tips for using the Q and A system? I'd like to continue using it if possible as I don't like the image type verifications. Thanks.