Classifying variable and $$xed manufacturing costs Classify each of the following as a vari$-$ able or $$xed cost with respect to a unit of production: $($a$)$ Salaries of production supervisors $($b$)$ Steel used in automobile production$($c$)$ Salaries for factory custodial sta$($d$)$ Depreciation of factory equipment $($e$)$ Lubricants for machines $($f$)$ Electricity used to operate a speci$$c machine $($g$)$ Hourly wages paid to production workers $($h$)$ Rent for a factory building $($i$)$ Glue used in furniture production $($j$)$ Maintenance for production equipment performed every month