Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ In an Ellingham diagram,the oxidation of carbon to carbon monoxide shows a negative slope with respect to temperature.
Reason $R:$ $CO$ tends to get decomposed at higher temperature.
In the light of the above statements,choose the correct answer from the options given below:

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