Which one of the following statements is not true?

  • A
    Clean water would have a $BOD$ value of $5 \ ppm$.
  • B
    Fluoride deficiency in drinking water is harmful. Soluble fluoride is often used to bring its concentration up to $1 \ ppm$.
  • C
    When the $pH$ of rain water is higher than $6.5$,it is called acid rain.
  • D
    Dissolved Oxygen $(DO)$ in cold water can reach a concentration up to $10 \ ppm$.

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$(2)$ Plastic materials $(B)$ Takes $7$ to $10$ months for natural decomposition
$(3)$ Agricultural waste $(C)$ Remains in original form for decades without decomposition

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State whether the following statements are true or false:
$(1)$ The concentration of oxygen in cold water is $100 \ ppm$.
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