Which of the following would be classified as a mixed cost for a T$-$shirt manufacturer?

a$.$ maintenance costs with sewing machine company contracted at $$2,000$ per year plus $$0\mathrm{.}001$ for each machine hour of use

b$.$ electricity costs of $$0\mathrm{.}038$ per kilowatt$-$hour

c$.$ property taxes on factory, building, and equipment

d$.$ hourly wages of sewing machine operators