The source of carbon is external source for

Options

A. Photoheterotrophs
B. Chemoautotrophs
C. Chemoheterotrophs
D. 1 & 3

Answer

To solve the question "The source of carbon is an external source for," we need to analyze the options provided and understand the definitions of the terms involved.

### Step-by-Step Solution:

1. **Understand the Terms**:
- **Photoheterotrophs**: These are organisms that use light as their primary energy source but cannot use carbon dioxide as their carbon source. Instead, they obtain carbon from organic compounds.
- **Chemoheterotrophs**: These organisms obtain both energy and carbon from organic compounds. They cannot utilize carbon dioxide as their carbon source.

2. **Analyze the Options**:
- **Option 1: Photoheterotrophs**: Since these organisms cannot use carbon dioxide and rely on external organic sources for carbon, they fit the criteria of having an external source of carbon.
- **Option 2: Chemoheterotrophs**: These organisms use organic compounds for both energy and carbon, so they also have an external source of carbon.
- **Option 3: Chemoheterotrophs**: This option appears to be a repetition of Option 2, which is also correct.
- **Option 4: One and Three**: This option suggests that both Photoheterotrophs (Option 1) and Chemoheterotrophs (Option 3) are correct.

3. **Determine the Correct Answer**:
- Since both Photoheterotrophs and Chemoheterotrophs rely on external sources for carbon, the correct answer is **Option 4: One and Three**.

### Final Answer:
The source of carbon is an external source for **Option 4: One and Three**.

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