Which of the following costs incurred by Alaska Manufacturing is an example of costs that are inseparable before the split$-$off point?

a$.$ Administrative costs of $$10,000$ for the corporate headquarters

b$.$ Support department costs of $$19,000$ for the Janitorial Department and the plant cafeteria

c$.$ Selling costs of $$40,000$ incurred by the Sales Department

d$.$ Joint costs of $$90,000$ incurred in the production process