A combination of lens is formed by keeping two identical equiconvex lens of focal length f kept in contact and the space between them filled with liquid of refractive index $\mu_2 =\large\frac{4}{3}$. Focal length of the combination is

Question

$(a)\;f \\ (b)\;\frac{f}{2} \\ (c)\;\frac{4}{3} f \\ (d)\;\frac{3f}{4} $