Maintenance costs for a regenerative thermal oxidizer increased according to an arithmetic gradient for 5 years. The cost in year 1 was $7000. If the interest rate is 10% per year and the present worth of the costs for a 5-year period was $28,800, the amount of the yearly increase, G, was closest to:

(a) $1670

(b) $945

(c) $620

(d) $330