Jay Vato works at Batwing Industries from midnight until 8:00 AM. His normal wage rate is $17 per hour, while Ben Phillips, who does the same job from 8:00 AM until 4:00 PM makes $15 per hour. Since Ben and Jay have the same seniority within the plant, the difference in pay is due to a(n):
a. overtime premium.
b. production bonus
c. make-up guarantee.
d. shift premium.