4. A plant asset was purchased on 1 January for $30,000 with an estimated residual value of $6,000 at the end of its useful life. 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 $15,000. The remaining useful life of the plant asset is: a. 10 years. b. 8 years. c. 5 years. d. 3 years. e. O years