Câu hỏi 7
John muốn thăm chín thành phố: Onecity, Twoland, Threecity, Fourland, Fivecity, Sixland, Sevencity, Eightland, Ninecity. Tuyến xe nối Threecity–Onecity, Ninecity–Twoland, Threecity–Ninecity, Ninecity–Onecity, Onecity–Twoland, Eightland–Ninecity, Sixland–Sevencity, Fivecity–Fourland, Fourland–Eightland. Xuất phát ở Threecity, cặp nào gồm hai thành phố không thể tới?