اذا قصدك السوال هذا
فجوابه خطاء

بالجوا بالصحيح هو b

,وهذا المرجع من المحاضرة 10

But in order to constitute a proper variable a good
categorisation/classification system should: be exhaustive,
have mutually exclusive categories, not mix categories of
different types in one set, have unambiguously defined