Microorganisms present in the polluted water use up the ______ present in it.

Options

A. oxygen
B. carbon
C. nitrogen
D. nitric acid

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Question**: The question asks what microorganisms in polluted water use up. This implies we need to identify a substance that is consumed by these microorganisms in the process of breaking down organic matter.

2. **Identifying the Context**: In polluted water bodies, there is a high amount of organic matter. Microorganisms, such as bacteria, thrive in these environments as they feed on this organic matter.

3. **Microbial Activity**: As these microorganisms digest the organic matter, they require energy to carry out their metabolic processes. This energy is obtained through respiration, which involves the consumption of oxygen.

4. **Impact on Water Quality**: The consumption of oxygen by microorganisms can lead to a decrease in the oxygen levels available for aquatic life, such as fish and plants. This is a significant concern in polluted water bodies.

5. **Biological Oxygen Demand (BOD)**: To measure the extent of oxygen consumption by microorganisms in polluted water, a parameter called Biological Oxygen Demand (BOD) is used. A high BOD indicates that a large amount of oxygen is being consumed by microorganisms, which is characteristic of polluted water.

6. **Conclusion**: Based on the understanding of the role of microorganisms in polluted water, it can be concluded that they primarily use up oxygen present in the water.

**Final Answer**: Microorganisms present in the polluted water use up the **oxygen** present in it.

---