The wavelength $\mathrm{\lambda }$ of the ${\mathrm{K}}_{\mathrm{a}}$ line of characteristic X-ray spectra varies with atomic number approximately

• A

  $\mathrm{\lambda }\propto \mathrm{Z}$

• B

  $\mathrm{\lambda }\propto \sqrt{\mathrm{Z}}$

• C

  $\mathrm{\lambda }\propto \frac{1}{{\mathrm{Z}}^2}$

• D

  $\mathrm{\lambda }\propto \frac{1}{\sqrt{\mathrm{Z}}}$