Silent killer gas is

Options

A. `SO_2`
B. `CO`
C. `CO_2`
D. Biogas

Explanation

**Step-by-Step Solution:**

1. **Identify the Question**: The question asks for the gas that is referred to as the "silent killer."

2. **Analyze the Options**: The options provided include:
- SO2 (Sulfur Dioxide)
- CO (Carbon Monoxide)
- CO2 (Carbon Dioxide)
- Biogas

3. **Evaluate SO2**: Sulfur Dioxide (SO2) is known to contribute to air pollution but is not typically referred to as a silent killer gas. It is irritating to the respiratory system and has a distinct smell.

4. **Evaluate CO**: Carbon Monoxide (CO) is a colorless, odorless gas that is non-irritating and can be very dangerous. It is often referred to as a "silent killer" because it can cause poisoning without any warning signs, such as smell or taste.

5. **Evaluate CO2**: Carbon Dioxide (CO2) is a gas that is used in processes like photosynthesis and is not considered a silent killer. It is a normal component of the atmosphere.

6. **Evaluate Biogas**: Biogas is generally considered eco-friendly and is not classified as a silent killer gas. It is produced from organic matter and is used as a renewable energy source.

7. **Conclusion**: Based on the evaluations, the only gas that fits the description of a "silent killer" due to its properties is Carbon Monoxide (CO).

**Final Answer**: The silent killer gas is Carbon Monoxide (CO).

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