$S_1$: If $-7$ is an integer,then $\sqrt{-7}$ is a complex number.
$S_2$: $-7$ is not an integer or $\sqrt{-7}$ is a complex number.

• A
  $S_1$ and $S_2$ are converse statements of each other.
• B
  $S_1$ and $S_2$ are negations of each other.
• C
  $S_1$ and $S_2$ are equivalent statements.
• D
  $S_1$ and $S_2$ are contrapositive of each other.