Each of Boggart$$s production managers $($annual salary cost, $$45,000)$ can oversee $60,000$ machine hours of manufacturing activity. Thus, if the company has $50,000$ hours of manufacturing activity, one manager is needed; for $75,000$ hours, two managers are needed; for $125,000$ hours, three managers are needed; and so forth. Boggart$$s salary cost can best be described as a:

A$.$ variable cost.

B$.$ step$-$fixed cost.

C$.$ semivariable cost.

D$.$ step$-$variable cost.

E$.$ fixed cost.