Solid targets of different elements are bombarded by highly energetic electron beams. The frequency (f) of the . characteristic X-rays emitted from different targets varies with atomic number Z as

• A

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

• B

  $\mathrm{f}\propto {\mathrm{Z}}^2$

• C

  $\mathrm{f}\propto \mathrm{Z}$

• D

  $\mathrm{f}\propto {\mathrm{Z}}^{3/2}$