Legume plants are important for atmosphere because that

Options

A. Help in `NO_(2)` fixation
B. Do not help in `NO_(2)` fixation
C. Increase soil fertility
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding Legume Plants**:
- Legume plants belong to the family Fabaceae. Common examples include beans, alfalfa, soybeans, and peanuts.

2. **Symbiotic Relationship**:
- Legume plants have a symbiotic relationship with certain bacteria, specifically Rhizobium, which reside in the root nodules of these plants.

3. **Nitrogen Fixation**:
- The Rhizobium bacteria are capable of fixing atmospheric nitrogen (N₂) into a form that the plants can use (ammonia). This process is crucial because atmospheric nitrogen is not directly usable by plants.

4. **Soil Fertility**:
- Through the process of nitrogen fixation, legume plants contribute to increasing soil fertility. The fixed nitrogen enriches the soil, making it more fertile for subsequent crops.

5. **Evaluating Options**:
- Now, let's evaluate the options given in the question:
- **Option 1**: They help in nitrogen dioxide fixation - This is incorrect; they fix nitrogen gas (N₂), not nitrogen dioxide (NO₂).
- **Option 2**: They do not help in nitrogen dioxide fixation - This is true but not the main reason for their importance.
- **Option 3**: They increase soil fertility - This is correct and is the main reason for their significance.
- **Option 4**: All of these - This cannot be true since the first option is incorrect.

6. **Conclusion**:
- The most appropriate answer is that legume plants are important for the atmosphere because they **increase soil fertility** through nitrogen fixation.

### Final Answer:
Legume plants are important for the atmosphere because they increase soil fertility.