Two similar thin equi-convex lenses, of focal length f each, are kept coaxially in contact with each other such that the focal length of the combination is $F_1$.When the space between the two lenses is filled with glycerin (which has the same refractive index $(\mu = 1.5)$ as that of glass) then the equivalent focal length is $F_2$. The ratio $F_1 : F_2$ will be :

Question

( A ) 2 : 1
( B ) 1 : 2
( C ) 3 : 4
( D ) 2 : 3