A product that is very labor-intensive assembled at Boyd's Aero Structure in Memphis has an average labor cost of $20/hr. Overhead expenses are charged at 100% of labor at this company. Time for the very first unit = 10 hours, time for the fourth unit = 8.1 hours. What is the cumulative time in hours for 100 units to be made?

a) 800 hours
b) 810 hours
c) 820 hours
d) 830 hours