For a Converging Lens with Two Curved Surfaces,the Radius of Curvature

For a converging lens with two curved surfaces,the radius of curvature for both surfaces is 11 cm.If the focal length is 20 cm,what must the index of refraction be?

A) 1.3
B) 1.6
C) 2.6
D) 1.7
E) ​1.5