Which one of the following gas was not present in free form at the time life originated on earth?

Options

A. Ammonia
B. Oxygen
C. Hydrogen
D. Methane

Answer

To solve the question "Which one of the following gases was not present in free form at the time life originated on earth?", we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Context**:
- The question is asking about the gases present in the atmosphere during the time life originated on Earth, which is approximately 3.5 billion years ago.

2. **Identify the Options**:
- The options provided are:
1. Ammonia
2. Oxygen
3. Hydrogen
4. Methane

3. **Review the Conditions of Early Earth**:
- At the time of Earth's formation (about 4.5 billion years ago), the atmosphere was very different from today. It was a reducing atmosphere, which means it lacked free oxygen.

4. **Analyze Each Gas**:
- **Ammonia (NH₃)**: This gas was likely present in the early atmosphere.
- **Oxygen (O₂)**: This gas was not present in free form during the early stages of Earth's atmosphere. It began to accumulate later due to photosynthetic organisms.
- **Hydrogen (H₂)**: This gas was also present in the early atmosphere.
- **Methane (CH₄)**: This gas was present in the early atmosphere as well.

5. **Conclusion**:
- Based on the analysis, the gas that was not present in free form at the time life originated on Earth is **Oxygen (O₂)**.

### Final Answer:
**Oxygen**

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