Correct answer: (A) Deoxygenation rate is proportional to remaining oxidizable organic matter.; (C) Reoxygenation rate is proportional to DO deficit.
Explanation
The classical model uses first-order BOD decay,$-dL/dt=k_dL$,and reaeration $r_a=k_a(C_s-C)=k_aD$. Thus A and C state the governing rate assumptions. Saturation concentration enters the oxygen deficit but does not replace L in deoxygenation. Select **A, C**.
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