If the maximum and minimum voltages of an $A.M.$ wave are $V_{\max}$ and $V_{\min}$ respectively,then the modulation factor '$m$' is

• A
  $\frac{(V_{\max} + V_{\min})}{(V_{\max} \cdot V_{\min})}$
• B
  $\frac{(V_{\max} - V_{\min})}{(V_{\max} + V_{\min})}$
• C
  $\frac{2 V_{\max} V_{\min}}{(V_{\max} + V_{\min})}$
• D
  $\frac{(V_{\max} + V_{\min})}{(V_{\max} - V_{\min})}$