A concave mirror of focal length f (in air) is immersed in water ($\mathrm{\mu } = \frac{4}{3}$). The focal length of the mirror in water will be 

• A

  $\mathrm{f}$

• B

  $\frac{4}{3}\mathrm{f}$

• C

  $\frac{3}{4}\mathrm{f}$

• D

  $\frac{7}{3}\mathrm{f}$