Which one of the following is not true ?

Question

$\begin{array}{1 1} \text{(a) Concentration of DO below 6 ppm is good for the growth of fish} \\\text{(b) Clean water would have a BOD value of less than 5 ppm} \\ \text{(c) Oxides of sulphur , nitrogen and carbon are the most widespread air } \\\text {(d) pH drinking water should be 5.5-9.5}\end{array}$