A convexo-concave convergent lens is made of glass of refractive index 1.5 and focal length `24 cm.` Radius of curvature for one surface is double than that of the other. Then,radii of curvature for the two surfaces are (in cm)
A. `6,12`
B. `12,24`
C. `3,6`
D. `18,36`