Electrons with de-Broglie wavelength $\lambda$  falls on the target in an x-ray tube. The cut- off wavelength of the emitted X-ray is 

• A

  $\frac{2{\mathrm{mc\lambda }}^2}{\mathrm{h}}$

• B

  $\frac{2\mathrm{h}}{\mathrm{mc}}$

• C

  $\frac{2{\mathrm{m}}^2{\mathrm{c}}^2{\mathrm{\lambda }}^2}{{\mathrm{h}}^2}$

• D

  $\mathrm{\lambda }$