Purification of drinking water in cities involves passing water through beds of ____ and ____

Explanation

To answer the question about the purification of drinking water in cities, we need to identify the materials used in the filtration process.

### Step-by-Step Solution:

1. **Understand the Process of Water Purification**:
- Water purification is essential to make sure that the water is safe for drinking. In cities, this process often involves multiple steps to remove impurities.

2. **Identify the Filtration Materials**:
- During the purification process, water is typically passed through different layers of materials that help in filtering out contaminants.

3. **Recognize Common Filtration Materials**:
- Two common materials used in the filtration of drinking water are **sand** and **gravel**.
- Sand helps to trap smaller particles and impurities, while gravel is used to support the sand layer and allows for better water flow.

4. **Formulate the Final Answer**:
- Therefore, the purification of drinking water in cities involves passing water through beds of **sand** and **gravel**.

### Final Answer:
The purification of drinking water in cities involves passing water through beds of **sand** and **gravel**.

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