The Terms 1^st Tier, 2^nd Tier, 3^rd Tier, in the Context

The terms 1^st tier, 2^nd tier, 3^rd tier, in the context of a supply chain refers to the ________.

A) levels of supplier certification
B) sequence of suppliers or customers that are in the chain but distant from the base/focal company
C) value categories of procured commodities
D) number of suppliers for a procured commodity