Which of the following items would likely be classified as a product cost in a plan that manufactures computer chips?

a. The silicon dioxide used to fabricate integrated circuits

b. Resistors and capacitors used in quality control testing.

c. The spare parts to fix a lithography system used to produce chips.

d. The plant's HR manager, responsible for maintaining records for all plant employees

e. Salaries for staff tasked solely with supervising production.