A is incorrect: It is indeed possible to add custom Python libraries to the Code Interpreter environment by uploading them as tool resources.

B is incorrect: A Function Tool allows the agent to call external code but does not integrate a Python library directly into the Code Interpreter's sandbox environment.

C is correct: Specialized or proprietary Python modules can be added to the Code Interpreter by uploading their respective package files (e.g., .whl files) as resource dependencies associated with the tool.

D is incorrect: While `requirements.txt` files are common for managing dependencies in Python projects, the specific mechanism for Code Interpreter customization involves uploading the package files directly as tool resources.