$A$ - Micro-organisms involved in the biodegradation of organic matter in the receiving water body consume a lot of oxygen,and as a result,there is a sharp decline in dissolved oxygen in the stream from the point of sewage discharge.
$R$ - $BOD$ decreases when the dissolved oxygen amount decreases in water.

• A
  $A$ and $R$ are correct.
• B
  $A$ and $R$ are incorrect.
• C
  $A$ is incorrect,$R$ is correct.
• D
  $A$ is correct,$R$ is incorrect.