Question 7
John wants to visit nine cities: Onecity, Twoland, Threecity, Fourland, Fivecity, Sixland, Sevencity, Eightland, Ninecity. Bus routes connect Threecity–Onecity, Ninecity–Twoland, Threecity–Ninecity, Ninecity–Onecity, Onecity–Twoland, Eightland–Ninecity, Sixland–Sevencity, Fivecity–Fourland and Fourland–Eightland. Starting at Threecity, which pair consists of cities he cannot visit?