Which of the following is not an example of mixed costs?

Question $1$Answer

a$.$

Equipment rental at $$15,000$ for first $10,000$ hours, $$1$ for each additional hour

b$.$

A sales base salary of $$75,000$ plus $$55$ for every unit sold

c$.$

$$15\mathrm{.}99$ for $5$G of data and $$\mathrm{.}99$ for each additional gigabyte

d$.$

Straight$-$line depreciation expense for warehouse equipment