Which of the following is a sink for `CO`?

Options

A. Haemoglobin
B. Microorganisms present in the soil
C. Oceans
D. Plants

Explanation

To determine which of the following options is a sink for carbon monoxide (CO), we need to understand what a "sink" means in this context. A sink is a natural or artificial system that absorbs or stores a substance—in this case, carbon monoxide.

### Step-by-Step Solution:

1. **Understanding Carbon Monoxide (CO)**:
- Carbon monoxide is a colorless, odorless gas that can be harmful to human health when inhaled. It binds to hemoglobin in the blood, preventing oxygen transport.

2. **Identifying Potential Sinks**:
- The options provided are:
- Hemoglobin
- Microorganisms present in soil
- Ocean
- Plants

3. **Analyzing Each Option**:
- **Hemoglobin**:
- While hemoglobin can bind to carbon monoxide, it does not act as a sink in the traditional sense. Instead, it can be dangerous because it forms carboxyhemoglobin, which reduces oxygen transport in the blood.
- **Microorganisms in Soil**:
- Microorganisms can metabolize carbon monoxide, effectively absorbing it and preventing it from accumulating in the atmosphere. Thus, they act as a natural sink for CO.
- **Ocean**:
- The ocean is primarily a sink for carbon dioxide (CO2) rather than carbon monoxide.
- **Plants**:
- Plants are also known to absorb carbon dioxide for photosynthesis but do not significantly absorb carbon monoxide.

4. **Conclusion**:
- Based on the analysis, the best answer is that microorganisms present in soil act as a natural sink for carbon monoxide.

### Final Answer:
Microorganisms present in soil act as a sink for carbon monoxide.

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