The wastewater from a city, containing a high concentration of biodegradable organics, is being steadily discharged into a flowing river at a location S. If the rate of aeration of the river water is lower than the rate of degradation of the organics, then the dissolved oxygen of the river water

• A. is lowest at the location S
• B. is lowest at a point upstream of the location S
• C. remains constant all along the length of the river
• D. is lowest at a point downstream of the location S