A certain city has a circular wall around it, and this wall has four gates pointing north,south, east and west. A house stands outside the city, three kms north of the north gate, and it can just be seen from a point nine kms east of the south gate. What is the diameter of the wall that surrounds the city?

• A. 6 km
• B. 9 km
• C. 12 km
• D. None of these