Two thin sysmmetrical lenses of different nature and of different material have equal raii of curvature `R=15cm`. The lenses are put close together and immersed in water `(mu_(w)=4//3)` . The focal length of the system ini water is 30cm. The difference between refractive indices of tthe two lenses is
A. `(1)/(2)`
B. `(1)/(4)`
C. `(1)/(3)`
D. `(3)/(4)`