Assertion : Atmospheric nitrogen is always fixed by nitrogen-fixing micro organisms.
Reason : They convert ammonia present in atmosphere to nitrites and nitrates.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion**: The assertion states that "Atmospheric nitrogen is always fixed by nitrogen-fixing microorganisms." This implies that all nitrogen fixation is solely dependent on microorganisms.

2. **Evaluating the Reason**: The reason given is "They convert ammonia present in atmosphere to nitrites and nitrates." This suggests that the role of nitrogen-fixing microorganisms is to convert ammonia into nitrites and nitrates.

3. **Clarifying Nitrogen Fixation**: Nitrogen fixation is the process where atmospheric nitrogen (N₂) is converted into ammonia (NH₃). This process is primarily carried out by certain bacteria, but it is important to note that not all nitrogen fixation is done by microorganisms. Approximately 70% of nitrogen fixation is done by microorganisms, while the remaining 30% occurs through photochemical and electrochemical processes.

4. **Distinguishing Between Nitrogen Fixation and Nitrification**: The assertion incorrectly states that nitrogen-fixing microorganisms convert ammonia to nitrites and nitrates. This process is actually called nitrification, which is different from nitrogen fixation. In nitrification, ammonia is oxidized to nitrites (NO₂⁻) and then to nitrates (NO₃⁻).

5. **Conclusion**: Since the assertion is incorrect (not all nitrogen fixation is done by microorganisms) and the reason is also incorrect (the reason describes nitrification, not nitrogen fixation), both the assertion and the reason are false.

**Final Answer**: Both assertion and reason are false.

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