Which of the following quotes best illustrates the idea of fixed cost?

a$."$As we increase output, per$-$unit costs fall."

b$."$My primary source of overhead cost is the cost of running the head office."

c$."$If I need to$,$ I can negotiate more overtime with my work force to meet unexpected orders."

d$."$If I double the number of workers and trucks, I get only $80$ percent more packages delivered."

e$.$ None of the above