Two lenses of power 6D and – 2D are combined to form a single lens. The focal length of this lens will be 

• A

  $\frac{3}{2}\mathrm{m}$

• B

  $\frac{1}{4}\mathrm{m}$

• C

  4 m

• D

  $\frac{1}{8}\mathrm{m}$