A thin lens made of glass of refractive index $\mathrm{\mu }$ = 1.5 has a focal length equal to 12 cm in air.  It is now immersed in water $\left(\mathrm{\mu }=\frac{4}{3}\right)$.  Its new focal length is 

• A

  48 cm 

• B

  36 cm

• C

  24 cm

• D

   12 cm