Gordon Drilling Co. purchases a driller for $14,000. Its market value for salvage purposes decreases 30% each year. When installed on an oil field, the machinery operates virtually all day, and operating and maintenance costs will be $3,500 the first year, increasing $600 each year thereafter. What is the optimum replacement interval if MARR is 15%?

a. 6 years

b. 5 years

c. 8 years

d. should never be replaced