In Ramsden eyepiece , the planoconvex lenses each of focal length f are separated by a distance 12 cm. the equivalent focal length (in cm) of the eyepiece is

Question

$$\begin {array} {1 1} (a)\;10.5 & \quad (b)\;12.0 \\ (c)\;13.5 & \quad (d)\;15.5 \end {array}$$