________ are geographic concentrations of interconnected companies and institutions in a particular field, which constitute a critical mass

Darius works in sales. For him, the most important thing is to build satisfying personal relationships with his co-workers and clients. Darius is primarily motivated by a need for ______.

Fill in the blank(s) with the appropriate word(s).

Has the group of students been sitting in the library all day?

A) students; been sitting B) group; been sitting C) students; sitting D) group; has been sitting