Which can't be eliminated by animals

Options

A. `NH_(3)`
B. urea
C. Free nitrogen
D. Uric acid

Answer

To solve the question "Which can't be eliminated by animals?" we will analyze the options provided: ammonia, urea, free nitrogen (gaseous nitrogen), and uric acid.

### Step-by-Step Solution:

1. **Understanding Excretory Products**:
- Animals produce various nitrogenous wastes as a result of protein metabolism. The main forms of nitrogenous waste include ammonia, urea, and uric acid.

2. **Identifying the Forms of Nitrogenous Waste**:
- **Ammonia**: Highly toxic and is excreted by aquatic animals in large amounts. These animals are termed ammonotelic.
- **Urea**: Less toxic than ammonia and is excreted by terrestrial animals. These animals are termed ureotelic.
- **Uric Acid**: Even less toxic and is excreted by birds and reptiles. These animals are termed uricotelic.

3. **Analyzing Free Nitrogen**:
- Free nitrogen, or gaseous nitrogen (N2), is a stable molecule that is not produced as a waste product in animal metabolism. It is not a byproduct of protein metabolism and is not excreted by animals.

4. **Conclusion**:
- Among the options provided, ammonia, urea, and uric acid are all excreted by animals. However, free nitrogen is not eliminated by any animal. Therefore, the answer to the question is **free nitrogen**.

### Final Answer:
Free nitrogen (gaseous nitrogen) cannot be eliminated by animals.

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