The focal length of an equiconvex lens is 20 cm and radius of curvature of each curved surface is 10 cm. one of the two curved surfaces is silvered. Find the effective focal length of the combination.

• A. \(\frac{{20}}{3}\) cm, concave
• B. 10 cm convex
• C. \(\frac{{10}}{3}\) cm, concave
• D. 10 cm concave