Given below are two statements with reference to the secondary treatment of sewage.
Statement $I$: The $BOD$ of effluent increases.
Statement $II$: Flocs of bacteria consume the major part of the organic matter present in the effluent.
In the light of the above statements,choose the most appropriate answer from the options given below.

• A
  Statement $I$ is correct and statement $II$ is incorrect.
• B
  Both statement $I$ and statement $II$ are incorrect.
• C
  Both statement $I$ and statement $II$ are correct.
• D
  Statement $I$ is incorrect and statement $II$ is correct.