Electrons with energy $80 \ keV$ are incident on the tungsten target of an $X$-ray tube. $K$-shell electrons of tungsten have an ionization energy of $72.5 \ keV$. The $X$-rays emitted by the tube contain only:

• A
  $A$ continuous $X$-ray spectrum (Bremsstrahlung) with a minimum wavelength of $\approx 0.155 \ \mathring{A}$
• B
  $A$ continuous $X$-ray spectrum (Bremsstrahlung) with all wavelengths
• C
  The characteristic $X$-ray spectrum of tungsten
• D
  $A$ continuous $X$-ray spectrum (Bremsstrahlung) with a minimum wavelength of $\approx 0.155 \ \mathring{A}$ and the characteristic $X$-ray spectrum of tungsten