C

The resistivity of an alloy typically does not follow simple linear relationships with the resistivities of its constituent metals. The resistivity of an alloy can be affected by its composition, structure, and the interaction between different metallic components.

In terms of the formulas: - If $R_{alloy} = R_{metal}$, it suggests that the alloy's resistivity is equal to that of a single metal, which is not typically valid for alloys.
- If $R_{alloy} < R_{metal}$ or $R_{alloy} > R_{metal}$, it would imply a clear relationship that does not generally hold true due to the complex interactions in alloys.

Hence, the most accurate statement is that there is no simple relation between the resistivity of the alloy and the resistivity of the constituent metals.