When originally purchased, a truck had an estimated useful life of seven years. The truck cost $68,000 and its estimated salvage value is $5,000. After five years of straight-line depreciation, the asset's total estimated useful life was revised from seven years to nine years, and there was no change in the estimated salvage value. What is the depreciation expense in year 6? A. $9,000 B. $7,000 C. $5, 750 D. $4, 500