Arrange the following in decreasing order (most important first) of their importance for the welfare of human society. Give reasons for your answer: biogas, citric acid, penicillin and curd.

Explanation

To arrange the products biogas, citric acid, penicillin, and curd in decreasing order of their importance for human welfare, we can analyze each product based on its impact on health, energy, nutrition, and preservation.

### Step-by-Step Solution:

1. **Penicillin**:
- **Importance**: Penicillin is the first discovered antibiotic and has revolutionized medicine. It is crucial for treating bacterial infections and has saved countless lives. It is effective against a wide range of diseases, including syphilis, gonorrhea, and diphtheria.
- **Conclusion**: Due to its significant impact on human health, penicillin is ranked first.

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