$A$: $A$ major part of the organic matter in water is consumed by chemical processes carried out by bacteria.
$R$: Microorganisms,including bacteria,consume the organic matter present in water.

• A
  $A$ and $R$ are both true and $R$ is the correct explanation of $A$.
• B
  $A$ and $R$ are both true,but $R$ is not the correct explanation of $A$.
• C
  $A$ is true and $R$ is false.
• D
  $A$ is false and $R$ is true.