Jack consultants bought a piece of equipment for $60,000. Its market value is defined by the relation $60,000-6000k, where k is the number of years. The equipment has an annual operating cost of $6,000 for the first 3 years, and increasing by $900 annually thereafter. If the equipment is projected to last 10 years, determine the economic service life at i=10%

A) 10 years

B) 6 years

C) 4 years

D) 8 years