A plant asset was purchased on January 1 for $360,000 with an estimated residual value of $60,000 at the end of its useful life. The current year's Depreciation Expense is $30,000 calculated on the straight-line basis and the balance of the Accumulated Depreciation account at the end of the year is $180,000. The remaining useful life of the plant asset is Select one: 10 years. O 6 years. 4 years. O 8 years