The measured BOD values for four different water samples (A-D) are as follows:
$\mathrm{A}=3 \mathrm{ppm}: \mathrm{B}=18 \mathrm{ppm}: \mathrm{C}-21 \mathrm{ppm}: \mathrm{D}=4 \mathrm{ppm}$. The water samples which can be called as highly polluted with organic wastes, are
