survey with two questions, both marked as required: have you tried to book rail travel online? [yes/no] were you successful? [yes/no] the second cannot be answered if the first is 'no', but the survey won't submit in this case and throws an error "please select one of these options"
https://bikeshed.vibber.net/system/media_attachments/files/109/829/514/876/428/560/original/c98670feade2a4b8.png