When two different materials A and B having atomic number $Z_1$ and $Z_2$ are used as the target in Coolidge $\gamma$-ray tube at different operating voltage $V_1$ and $V_2$ respectively their spectrums are found as below.



The correct relation is

• A

  $V_1>V_2$ and ${ Z}_1>Z_2$
• B

  $V_1<V_2$ and $Z_1<Z_2$
• C

  $V_1<V_2$ and $Z_1>Z_2$
• Correct answer
  D

  $V_1>V_2$ and $Z_1<Z_2$

Hence, the correct answer is option: 'D'.