Which Statement Illustrates the Concept of External Cost

Which statement illustrates the concept of external cost?

A) Margaret purchases all her food and clothing in the big city outside her residence.
B) A small business owner frequently buys raw materials by using her bank's line of credit.
C) Raymond cannot open his windows at times because he lives downwind from a mushroom farm.
D) Felicia, an economics major, asks the most insightful questions in class.