The radius of lower base of a right circular cone is 10 cm and the height is 10 cm. If a plane parallel to its base cuts the cone at a height of 6 cm from the base and the volume of frustum of cone is 312 π cm3 then, the radius on top of frustum of cone is,

• A. 6 cm
• B. 4 cm
• C. 3 cm
• D. 2 cm