A thin bi-convex lens of focal length 20 cm is made of glass of refractive index 1.5, when it is dipped in a liquid of refractive index $\large\frac{9}{8}$, it acts as :

Question

$(a)\;Convex\;lens\;of\;focal\;length\;15 cm \\(b)\;Convcave\;lens\;of\;focal\;length\;30 cm \\(c)\;Convex\;lens\;of\;focal\;length\;30 cm \\(d)\;Concave\;lens\;of\;focal\;length\;15 cm$