Which of the following costs is best classified as a fixed cost with respect to volume of activity?

Which of the following costs is best classified as a fixed cost with respect to volume of activity?

Total cost of salaries paid to quality inspectors in a manufacturing plant

Cleaning supplies used in an automobile assembly plant

Straight-line depreciation expense of a machine used in a factory

Tires used in an automobile manufacturing plant

Direct labor

Electricity used to heat, light, and cool a factory