$\Delta \lambda$ is the difference between the wavelength of the $K_{\alpha}$ line and the minimum wavelength of the continuous $X$-ray spectrum when the $X$-ray tube is operated at a voltage $V$. If the operating voltage is changed to $V/3$,then the above difference is $\Delta \lambda^{\prime}$. Then:

• A
  $\Delta \lambda^{\prime} = 5 \Delta \lambda$
• B
  $\Delta \lambda^{\prime} = 4 \Delta \lambda$
• C
  $\Delta \lambda^{\prime} = 3 \Delta \lambda$
• D
  $\Delta \lambda^{\prime} < 3 \Delta \lambda$