Classify the following costs as either a product cost or a period cost:

a. Direct materials used
b. Factory utilities
c. Salespersons' commissions
d. Salary of plant manager
e. Indirect materials used
f. Depreciation on store equipment
g. Indirect labor incurred
h. Advertising expense
i. Direct labor incurred
j. Factory machinery repairs and maintenance
k. Depreciation on factory machinery
l. Plant insurance expired