In a simple microscope, if the final image is located at infinity then its magnifying power is 

• A

  $\frac{25}{\mathrm{f}}$

• B

  $\frac{\mathrm{D}}{26}$

• C

  $\frac{\mathrm{f}}{25}$

• D

  $\frac{\mathrm{f}}{\mathrm{D}+1}$