A waste water stream $\left(\mathrm{flow}=2 \mathrm{~m}^{3} / \mathrm{s},\right.$ ultimate $\left.\mathrm{BOD}=90 \mathrm{mg} / 1\right)$ is joining a small river $(\mathrm{flow}=$ $12 \mathrm{~m}^{3} / \mathrm{s},$ ultimate $\mathrm{BOD}=5 \mathrm{mg} / 1$ ). Both water streams get mixed up instantaneously. Crosssectional area of the river is $50 \mathrm{~m}^{2}$. Assuming the de-oxygenation rate constant, $\mathrm{k}^{\prime}=0.25 /$ day, the $\mathrm{BOD}$ (in $\mathrm{mg} / 1$ ) of the river water, $10 \mathrm{~km}$ downstream of the mixing point is
(A) 1.68
(B) 12.63
(C) 15.46
(D) 1.37
A waste water stream $\left(\mathrm{flow}=2 \mathrm{~m}^{3} / \mathrm{s},\right.$ ultimate $\left.\mathrm{BOD}=90 \mathrm{mg} / 1\right)$ is joining a small river $(\mathrm{flow}=$ $12 \mathrm{~m}^{3} / \mathrm{s},$ ultimate $\mathrm{BOD}=5 \mathrm{mg} / 1$ ). Both water streams get mixed up instantaneously. Crosssectional area of the river is $50 \mathrm{~m}^{2}$. Assuming the de-oxygenation rate constant, $\mathrm{k}^{\prime}=0.25 /$ day, the $\mathrm{BOD}$ (in $\mathrm{mg} / 1$ ) of the river water, $10 \mathrm{~km}$ downstream of the mixing point is
(A) 1.68
(B) 12.63
(C) 15.46
(D) 1.37