Given below are two statements about $X$-ray spectra of elements :
Statement $(I) :$ $A$ plot of $\sqrt{\nu}$ ($\nu =$ frequency of $X$-rays emitted) vs atomic mass is a straight line.
Statement $(II) :$ $A$ plot of $\nu$ ($\nu =$ frequency of $X$-rays emitted) vs atomic number is a straight line.
In the light of the above statements choose the correct answer from the options given below :

• A
  Statement $I$ is true but Statement $II$ is false
• B
  Both Statement $I$ and Statement $II$ are true
• C
  Both Statement $I$ and Statement $II$ are false
• D
  Statement $I$ is false but Statement $II$ is true