A product has to go through 4 stages before it turns into a finished good. The outputs of 4 stages in units per hour are given below: Stage 1: 15, Stage 2: 15, Stage 3: 12, Stage 4: 15. What is the total output per hour for all four stages?
a) 57 units per hour.
b) 57 units per day.
c) 57 units per week.
d) 57 units per month.