It Has Been Suggested That a Rigid Definition for a Category

Multiple Choice

It has been suggested that a rigid definition for a category is not possible and that resemblance (much like a family resemblance) may be more appropriate. Why is this the case?

A) Categorization is very often a matter of degree, not an all-or-none process.
B) Categories constantly add new members.
C) Similarity is often subjective.
D) A rigid definition is unlikely to be accepted by everyone.