Equipment was purchased on January 1 for $39,000 with an estimated residual value of $3,000 at the end of its useful life. The current year's Depreciation Expense is $4,000 calculated on the straight-line basis and the balance of the Accumulated Depreciation account at the end of the year is $12,000. The remaining useful life of the equipment is: 38 3 years b. 9 years 5 years d. 6 years