A plant asset was purchased on January 1 for $90000 with an estimated salvage value of $20000 at the end of its useful life. The current year's Depreciation Expense is $10000 calculated on the straight-line basis and the balance of the Accumulated Depreciation account at the end of the year is $40000. The remaining useful life of the plant asset is 3 years. 9 years. 7 years. 4 years