If you climb mountains, you will not live to a ripe old age. But you will be bored unless you climb mountains. Theref...

Is "But" the reason that unless did not signify a necessary condition this time? Or the lack of negation in the first half? so confused and I can't seem to pinpoint why I did this wrong. I diagrammed: B ---> not CM CM ---> not B as opposed to what was used in the video. None of the questions answered this truly. -Annelise