What is the difference between a product cost and a period cost?: $*$

a$.$ Product costs can be associated with production but period costs are not and are recognized in the period incurred.

b$.$ Period costs are associated with production while product costs are expensed as incurred.

c$.$ Both period and product costs are associated with production and added to the cost of inventory.

d$.$ Product costs and period costs are both assumed to be expensed as incurred.