Assertion $A$: $SF_6$ is highly stable.
Reason $R$: $SF_6$ is a gas.
The correct option among the following is:

• A
  $A$ is true, $R$ is true and $R$ is the correct explanation for $A$
• B
  $A$ is true, $R$ is true but $R$ is not the correct explanation for $A$
• C
  $A$ is true but $R$ is false
• D
  $A$ is false but $R$ is true