`N_2` fixation is

Options

A. `N_2toNH_3`
B. `N_2toNO_3`
C. `N_2to` Amino acid
D. Both (a) and (b)

Explanation

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**: Nitrogen fixation is the process by which atmospheric nitrogen gas (N₂) is converted into a more usable form for living organisms, such as ammonia (NH₃) or nitrates (NO₃⁻).

2. **Sources of Nitrogen Fixation**: This process can occur through several mechanisms:
- **Biological Nitrogen Fixation**: Certain bacteria and fungi can convert atmospheric nitrogen into ammonia. Examples include Rhizobium bacteria that associate with leguminous plants.
- **Physical Processes**: Nitrogen can also be fixed through natural phenomena such as lightning, which can convert N₂ into nitrates.

3. **Chemical Conversion**: The main chemical reactions involved in nitrogen fixation include:
- Conversion of N₂ to NH₃ (ammonia).
- Further conversion of NH₃ to NO₃⁻ (nitrate), which is also a usable form of nitrogen for plants.

4. **Identifying the Correct Answer**: Among the options provided, the correct interpretation of nitrogen fixation is that it involves the conversion of nitrogen gas (N₂) into ammonia (NH₃) or nitrates (NO₃⁻). Therefore, the answer that encompasses these conversions is option D: "Nitrogen is converted into ammonia or nitrates."

### Final Answer:
**Nitrogen fixation is the process where atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or nitrates (NO₃⁻), making it usable for organisms. The correct answer is option D.**

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