For a Diverging Lens with One Flat Surface,the Radius of Curvature

For a diverging lens with one flat surface,the radius of curvature for the curved surface is 20.0 cm.What must the index of refraction be so that the focal length is -15.0 cm?

A) 0.333
B) 1.33
C) 2.33
D) 5.00