Which of the following can not act as an adsorbent? 

Options

A. Silica gel
B. Clay
C. Oxygen gas
D. Activated charcoal

Answer

To determine which of the following substances cannot act as an adsorbent, we need to understand the concept of adsorption and the properties of adsorbents.

### Step-by-Step Solution:

1. **Understanding Adsorption**:
- Adsorption is the process where molecules adhere to the surface of a solid or liquid. This is different from absorption, where molecules penetrate into the bulk of a solid or liquid.

2. **Identifying Adsorbents**:
- Common adsorbents include materials like silica gel, activated charcoal, and clay. These materials have high surface areas and can attract and hold molecules on their surfaces.

3. **Analyzing the Options**:
- We need to evaluate each option provided in the question to see if they can act as adsorbents.
- **Silica Gel**: This is a well-known adsorbent used for drying and moisture control.
- **Clay**: Clay can also act as an adsorbent due to its fine particles and surface area.
- **Activated Charcoal**: This is another effective adsorbent used in various applications, including water purification.
- **Oxygen Gas**: Unlike the other materials, oxygen is a gas and does not have a surface area to adsorb other molecules. It does not adhere to surfaces in the same way that solids do.

4. **Conclusion**:
- Based on the analysis, the substance that cannot act as an adsorbent is **oxygen gas**. Therefore, the answer to the question is option number three (if oxygen gas is the third option).

### Final Answer:
**Oxygen gas cannot act as an adsorbent.**

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