If A, B, Y and Z all contain the same instance data d1, then d1 should be declared in X and inherited into Y, Z, A and B.

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Quizplus · Accepted Answer

Anything in common between A, B, Y and Z should be declared in X to properly identify that it is a class trait common amongst them. That is, common elements should be defined as high up in the hierarchy as possible. If all descendants of X have the trait (instance data) d1, then it is part of X too.

If A, B, Y and Z All Contain the Same