When Identifying the Proper Instance Variables to Use in the Design

When identifying the proper instance variables to use in the design of an inheritance hierarchy, how do you decide where in the hierarchy a variable should be placed?

A) Instance variables should only be declared in the superclass and then accessed using the super keyword.
B) Instance variables should only be declared in subclasses.
C) Instance variables that are common to all classes should be placed at the base of the hierarchy.
D) Instance variables that are relevant to only one class should be placed at the base of the hierarchy.