Response to the following problem:
Ariel builds innovative loudspeakers for music and home theater. Identify the following costs as variable or fixed:
a. Depreciation on routers used to cut wood enclosures e. Grill cloth
b. Wood for speaker enclosures f. Glue
c. Patents on crossover relays g. Quality inspector's salary
d. Crossover relays
Support your answer using a minimum of two professional or academic sources.