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 fluolide is often used to bring its concentration upto 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 concentlation upto 10 ppm.
