Suppose I have BaseClass with public methods A and B, and I create DerivedClass through inheritance.
e.g.
public DerivedClass : BaseClass {} <
public DerivedClass : BaseClass {}
If they're defined public in the original class, you cannot override them to be private in your derived class. However, you could make the public method throw an exception and implement your own private function.
Edit: Jorge Ferreira is correct.