The graph shows how the inverse of magnification $\frac{1}{\mathrm{m}}$  produced by a thin convex lens varies with object distance u. What was the focal length of the lens used?     

 

• A

  $\frac{\mathrm{b}}{\mathrm{c}}$

• B

  $\frac{\mathrm{b}}{\mathrm{ca}}$

• C

  $\frac{\mathrm{bc}}{\mathrm{a}}$

• D

  $\frac{\mathrm{c}}{\mathrm{b}}$