Both $X$-rays and $\gamma$-rays are electromagnetic waves. Which of the following statements is true for them?

• A
  The energy of $X$-rays is greater than that of $\gamma$-rays.
• B
  Generally,the wavelength of $X$-rays is greater than that of $\gamma$-rays.
• C
  The frequency of $X$-rays is greater than that of $\gamma$-rays.
• D
  The velocity of $X$-rays is greater than that of $\gamma$-rays.