A roller conveyor system used to transport cardboard boxes along an order-filling line costs $100,000 plus $20,000 to install. It is estimated to depreciate at a declining-balance rate of 25 percent per year over its 15-year useful life. Annual maintenance costs are estimated to be $6000 for the first year, increasing by 20 percent every year thereafter. In addition, every third year, the rollers must be replaced at a cost of $7000. Interest is at 10 percent. What is the economic life of the roller conveyor system? Oa 10 years b. 6 years oc. 9 years d. 8 years