Which of the following BEST illustrates a bonus?

A$.$ An employee is compensated a fixed amount on a monthly basis.

B$.$ An employee receives extra pay for working in excess of $40$ hours per week.

C$.$ An employee reaches a sales goal and receives extra pay.

D$.$ An employee earns pay for hours worked.

E$.$ An employee is paid more when she learns new skills and becomes proficient at a different job.