请帮我解决一下问题~~
                                                                                                                                                                                                                                                                                                                        A company has just purchased a new machine, costing £150,000, for a contract. It has an installation cost of
£25,000 and is expected to have a scrap value of £10,000 in five years’ time. The machine will be depreciated on
a straight line basis over five years.
What is the relevant cost of the machine for the contract?
A £140,000
B £150,000
C £165,000
D £175,000

这道题答案是 C。
我在想 为什么150，000 不作为sunk cost 处理呢，怎么还会成为相关成本？
从答案上来看是 150，000+25，000-10，000 为啥那个残值要减掉？
谢谢大家~