In an interference experiment,let $D$ be the distance between the slit and the eyepiece. When the distance between two virtual sources is changed from $d_{A}$ to $d_{B}$,the fringe width changes from $Z_{A}$ to $Z_{B}$. The ratio $Z_{A} / Z_{B}$ is:

• A
  $\left(\frac{d_{A}}{d_{B}}\right)^{2}$
• B
  $\left(\frac{d_{A}}{d_{B}}\right)$
• C
  $\left(\frac{d_{B}}{d_{A}}\right)$
• D
  $\sqrt{\frac{d_{B}}{d_{A}}}$