Brock was describing the inside of his doctor's office to one of his friends. In his description, he mentions that there were two diplomas on the wall, even though this doctor does not have any diplomas displayed. Brock's error in recall illustrates

a$.$ the need for conceptual hierarchies in long$-$term memory.

b$.$ the need for a good executive control system in short$-$term memory.

c$.$ the role of schemas in long$-$term memory.

d$.$ the role of semantic networks in long$-$term memory.