While shopping at a grocery store, Jack is trying to decide which cereal to buy. He likes all types of Cheerios yet is pretty indifferent toward Chex. He also thinks Trix is a cereal for kids, so he does not even consider that brand. For Jack, Chex represents which specific type of set based on his awareness of cereals?

a$.$

Inert set

b$.$

Inept set

c$.$

External set