Equipment was purchased on January 1 for $39,000 with an estimated residual value of $3,000. The current year's Depreciation Expense is $3,000, 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 1) 3 years. 2) 8 years. 3) 6 years. 4) 5 years