Arrange the following in the 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 given items—Biogas, Citric acid, Penicillin, and Curd—in decreasing order of their importance for the welfare of human society, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Importance of Each Item**:
- **Penicillin**: This is an antibiotic that plays a crucial role in treating bacterial infections. It saves lives by effectively killing pathogens that cause diseases.
- **Biogas**: This is a renewable energy source produced from organic waste. It serves as a clean fuel for cooking and electricity generation, thus reducing pollution and reliance on fossil fuels.
- **Curd**: Curd is a fermented dairy product that contains beneficial bacteria (lactobacillus). It enhances nutritional value by providing essential vitamins like B12 and helps maintain gut health by replacing harmful bacteria with beneficial ones.
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