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-$1$: Sky wave signals are used for long-distance radio communication. These signals are, in general, less stable than ground wave signals.
Statement-$2$: The state of the ionosphere varies from hour to hour, day to day, and season to season.

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