___________cannot fix atmospheric nitrogen in the soil.
(Rhizobium/ Clostridium/Azotobacter/Penicillin)

Explanation

To solve the question, "___________ cannot fix atmospheric nitrogen in the soil," we need to analyze the options provided: Rhizobium, Clostridium, Azotobacter, and Penicillin.

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process by which certain bacteria convert atmospheric nitrogen (N₂) into a form that plants can use, such as ammonia (NH₃).

2. **Evaluating the Options**:
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