In the oil wells, X forms the topmost layer, followed by Y and finally Z. X, Y and Z are respectively

Options

A. Natural gas, oil and water
B. Petroleum gas, oil and natural gas
C. Natural gas, water and oil
D. Petroleum gas, oil and water.

Explanation

To solve the question regarding the layers in oil wells, we need to identify the substances represented by X, Y, and Z based on their properties and positions in the oil well.

### Step-by-Step Solution:

1. **Understanding the Layers in Oil Wells:**
- In oil wells, there are three distinct layers formed due to the differences in density of the substances present.

2. **Identifying the Topmost Layer (X):**
- The topmost layer (X) is typically composed of **natural gas**. Natural gas is lighter than both oil and water, which allows it to rise to the top.

3. **Identifying the Middle Layer (Y):**
- The middle layer (Y) consists of **petroleum oil**. Oil is less dense than water but denser than natural gas, so it sits below the natural gas layer.

4. **Identifying the Bottom Layer (Z):**
- The bottom layer (Z) is made up of **water**. Water is the densest of the three substances, which is why it forms the bottom layer in the oil well.

5. **Final Arrangement:**
- Therefore, the arrangement of layers from top to bottom is:
- X: Natural Gas
- Y: Petroleum Oil
- Z: Water

### Conclusion:
- The substances X, Y, and Z are respectively **Natural Gas (X)**, **Petroleum Oil (Y)**, and **Water (Z)**.