Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: In the Ellingham diagram,a sharp change in slope of the line is observed for $Mg \rightarrow MgO$ at $\sim 1120^{\circ} C$.
Reason $R$: There is a large change of entropy associated with the change of state.
In the light of the above statements,choose the correct answer from the options given below.

• A
  Both $A$ and $R$ are true but $R$ is $NOT$ the correct explanation of $A$.
• B
  Both $A$ and $R$ are true and $R$ is the correct explanation of $A$.
• C
  $A$ is false but $R$ is true.
• D
  $A$ is true but $R$ is false.