Suppose a firm is currently producing $20$ units of output $($Q$).$ If the average fixed cost incurred at that level of output is $$2\mathrm{.}50,$ then what must be this firm's total fixed cost?

a$.$

$$0$; the firm is not incurring a fixed cost

b$.$

between $$0$ and $$30$

c$.$

between $$30$ and $$60$

d$.$

between $$60$ and $$90$

e$.$

greater than $$90$