This question has Statement $-1$ and Statement $-2.$ Of the four choices given after the Statements,choose the one that best describes the two Statements.
Statement $-1:$ Short wave transmission is achieved due to the total internal reflection of the $e-m$ wave from an appropriate height in the ionosphere.
Statement $-2:$ Refractive index of the ionosphere is independent of the frequency of $e-m$ waves.

• A
  Statement $-1$ is true,Statement $-2$ is false.
• B
  Statement $-1$ is false,Statement $-2$ is true.
• C
  Statement $-1$ is true,Statement $-2$ is true but Statement $-2$ is not the correct explanation of Statement $-1$.
• D
  Statement $-1$ is true,Statement $-2$ is true and Statement $-2$ is the correct explanation of Statement $-1$.