Which is not an example of Mixed Costs? $(1$pt$)$

Maintenance Department with parts $($based on usage$)$ and salary costs

Rent for a building where utility charges $($based on usage$)$ and rent charges are on the same invoice

Mortgage payments, where total owed per month is going down this month due to refinancing

CPA firm wages with full$-$time salaried and part$-$time hourly employees