Electrons of mass  m  with de-Broglie wavelength $\lambda$ fall on the target in an X -ray tube. The cutoff wavelength $\left({\lambda }_0\right)$ of the emitted  X -ray is

• A

  ${\lambda }_0=\frac{2mc{\lambda }^2}{h}$

• B

  ${\lambda }_0=\frac{2h}{mc}$

• C

  ${\lambda }_0=\frac{2m^2{c}^2{\lambda }^3}{h^2}$

• D

  ${\lambda }_0=\lambda$