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What does "formally undecidable propositions" refer to in the context of Principia Mathematica and related systems?
A) Propositions that can be proven to be true or false within the system.
B) Propositions that can be decided using empirical methods outside the formal system.
C) Propositions that cannot be proven true or false within the formal system itself, regardless of the methods used.
D) Propositions that are proven to be false within the formal system.