问题如下：
Alex bought a machine with $50,000 in 2016, the useful life of this machine is ten years . The residual value of it is $5,000. If we use double-declining balance method, the depreciation of this machine in 2016 is:
选项：
A.$5,000
B.$4,500
C.$10,000
解释：
C is correct. 50,000 × 20 percent=10,000
为什么不用500,000-5,000？残值不用算进去吗？
