Which one of the following statements is not true?

• A
  $pH$ of drinking water should be between $5.5 - 9.5$.
• B
  Concentration of $DO$ below $6 \ ppm$ is good for the growth of fish.
• C
  Clean water would have a $BOD$ value of less than $5 \ ppm$.
• D
  Oxides of sulphur,nitrogen and carbon are the most widespread air pollutants.