Shown in the figure here is a convergent lens placed inside a cell filled with a liquid. The lens has focal length `+ 20 cm` when in air and its material has refractive index 1.50. If the liquid has refractive index 1.60, the focal length of the system is
A. `+ 80 cm`
B. `- 80 cm`
C. `- 24 cm`
D. `-100 cm`
