overriding members?

[NaNg15241]

This is my problem: I have these classes: public Class C { } public abstract Class A { protected C; } public Class D : C { } public Class B : A { private D; } When I do this: B = new B();, I have D AND C in B. And I need ONLY ONE, which is the newer one - D. How can I make the Class A use the new one (D) insted of the old one?