What Greek mathematician came up with the idea of the octave and first focused on the pleasing sounds that some notes make together

Spencer's blog claims that national parks are the best vacation destinations. which evidence is best for developing his claim? a personal account from a park visitor who describes positive adventures at grand canyon and yellowstone parks an interview with a travel agent who advertises cruises, eco-tours, and whitewater rafting trips a chart that compares the number of national parks in the eastern united states to that in the western states a study that proves adults who take yearly vacations have lower blood pressure than those who do not