Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R:$
Assertion $A:$ Polluted water may have a value of $BOD$ of the order of $17 \, ppm$.
Reason $R:$ $BOD$ is a measure of oxygen required to oxidise both the biodegradable and non-biodegradable organic material in water.
In the light of the above statements,choose the most appropriate answer from the options given below.

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