Thanks for taking your time and typed everthing.
Ya keith i know very well about referencetype and object type and the overriding will look only RHS objecttype for methods.
The one thing that i got confused is in your statement you used instance in place of referenceType.
if you call the instance a Three_dimensinal or B or A, the overridden method in Three_dimensinal will be called.
My understanding is instance will mean only the object type.Am i correct or people call the referencetype too as instance( like the quote statement )