Centralized design is $\_\_\_\_\_.$

a$.$ a process by which all database design decisions are carried out centrally by a large group of people

b$.$ a process in which conceptual design models subsets of an organization's database requirements, which are then aggregated into a complete design.

c$.$ suitable in a top$-$down approach when the problem domain is relatively small, as in a single unit or department in an organization

d$.$ often used when the problem itself is spread across several operational sites and each element is a subset of the entire data