问题如下：
Company M purchased a $400,000 equipment with a 3-year useful life at the beginning of 2013, considering the salvage value is zero. The equipment was depreciated on double-declining balance method throughout its useful life. If compamy M shifts to straight-line method at the beginning of 2014, the annual depreciation expense of 2015 tends to be:
选项：
A. stay the same
B. smaller than before
C. greater than before
解释：
C is correct. Under double-declining balance method, the depreciation expense is the highest in the first year, then the expense tends to decline. If company M shifts to straight-line method at the beginning of 2014, the depreciation expense of 2014 and 2015 will be equal, which means the depreciation expense of 2014 is lower than before and that of 2015 is higher than before.
算一遍时间很长，有简单的思考方法吗