Choose the factor that increases the BOD of water supply:

Explanation

The correct answer is option A: CO2. When the levels of CO2 increase, it can stimulate bacterial growth, leading to higher BOD values in the water supply. Options B, C, and D are incorrect because ozone (Option B) can actually help reduce organic pollutants, water (Option C) does not directly impact BOD levels, and the impact of ethanol (Option D) on BOD levels is variable depending on concentration and water composition.