关键是确定什么组成固定成本，如电话机的成本，和变动成本，如每打一个电话的成本。

The key is to determine what makes up fixed costs, such as the cost of a telephone, and variable costs, such as the cost of each phone call.

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用一根连线把两个罐头盒串起来，线的两端各自固定在每个罐头盒底部的小孔上，拉紧以后，就做成了相距几码远的很好的一部电话机。

Two tins, connected with string fixed through a hole in the bottom of each tin and stretched tight, form a good telephone over a few yards.

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