The wave length $\lambda$ of the $K_{\alpha }$ line of characteristic x-ray spectra varies with atomic number approximately

• A

  $\lambda \infty Z$

• B

  $\lambda \infty \sqrt{Z}$

• C

  $\lambda \infty \frac{1}{Z^2}$

• D

  $\lambda \infty \frac{1}{\sqrt{Z}}$