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:

A

1 : 2

B

2 : 3

C

3 : 4

D

2 : 1