The radius of the base of a 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?

• A. 1 ∶ 4 ∶ 8
• B. 1 ∶ 3 ∶ 5
• C. 1 ∶ 3 ∶ 9
• D. 1 ∶ 6 ∶ 12