The radius of base of solid cone is 9 cm and its height is 21 cm. It cut into 3 parts by two cuts, which are parallel to its base. The cuts are at height of 7 cm and 14 cm from the base respectively. What is the ratio of curved surface areas of top, middle and bottom parts respectively?

• 1 : 4 : 8
• 1 : 3 : 5
• 1 : 3 : 9
• 1 : 6 : 12