Assertion $(S)$: Flocs are organisms that consume inorganic substances in water.
Reason $(R)$: $STPs$ are used to prevent water pollution.

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