What is the requirement for a class to be used as a custom Visualforce controller?

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What is the requirement for a class to be used as a custom Visualforce controller?
A . Any top-level Apex class that has a constructor that returns a PageReference
B . Any top-level Apex class that extends a PageReference
C . Any top-level Apex class that has a default, no-argument constructor
D . Any top-level Apex class that implements the controller interface

Answer: D