Exclusive Answers on Checkboxes

If you have any questions that include Checkboxes (qualifying questions, drill downs, or plain old checkboxes), you can specify one ore more answers that, when checked, prohibit other answers from being checked.

The way you do that is a bit hidden/very hidden, but rollover an answer and'll you see the exclusive icon show up in grey. Click it to turn it blue and then that answer will be exclusive. You'll then see a warning about that below.