Hector spent the last three months on a winter fishing boat in Alaska. He was doing research on population levels of fish for conservation efforts. Hector most likely works in Plant Systems.
Agribusiness Systems.
Natural Resource Systems.
Environmental Service Systems

throughout history, economists have always thought  ..

i think that the answer is a

basic principle: a pronoun usually refers to something earlier in the text (its antecedent) and must agree in number — singular/plural — with the thing to which it refers. the indefinite pronouns anyone, anybody, everyone, everybody, someone, somebody, no one, and nobody are always singular.