老师C选项是怎么和公式对应起来的，求解

问题如下：

Company M purchased a $400,000 equipment with a 4-year useful life at the beginning of 2016, considering the salvage value is zero. In the first year of its useful life, the units of production accounted for 23% of the equipment’s lifetime production capacity. If company M wants its operating profit to be the highest in the first year of use, which depreciation method should it adopt?

选项：

double-declining balance.

straight line.

units of production.

解释：

C is correct. The lower the depreciation expense is, the higher the operating profit will be.

Depreciation expense:

Straight line = $400,000 /4 = ? $100,000

Double-declining balance = $400,000 × (2/4) = $200,000

Units of production = $400,000 × 0.23 = ? $92,000

So company M should adopt the units of production method.