A source of nonbiological carbon dioxide is

Options

A. volcanic eruptions
B. hot springs
C. weathering
D. all the above

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for a source of nonbiological carbon dioxide. Nonbiological sources refer to those that do not originate from living organisms.

2. **Identifying Potential Sources**: We need to consider various natural processes that release carbon dioxide into the atmosphere. Common sources include volcanic eruptions, hot springs, and weathering of rocks.

3. **Analyzing Volcanic Eruptions**: Volcanic eruptions are significant sources of nonbiological carbon dioxide. During an eruption, magma is expelled, which contains a large amount of carbon dioxide. When the magma cools, it can also produce smoke that contains carbon dioxide.

4. **Considering Hot Springs**: Hot springs do release carbon dioxide, but they primarily contain sulfur and are not considered a major source of nonbiological carbon dioxide compared to volcanic eruptions.

5. **Evaluating Weathering**: Weathering of rocks can release carbon dioxide, but this process is often influenced by biological factors and is not a direct source like volcanic eruptions.

6. **Conclusion**: Based on the analysis, the most significant source of nonbiological carbon dioxide is volcanic eruptions.

### Final Answer:
The source of nonbiological carbon dioxide is **volcanic eruptions**.

---