A achromatic combination is made with a lens of focal length   and dispersive power $\mathrm{\omega }$ with a lens having dispersive power of 2$\mathrm{\omega }$. The focal length of second will be

• A

  2 f 

• B

  $\frac{\mathrm{f}}{2}$

• C

  $-\frac{\mathrm{f}}{2}$

• D

  -2f