Classify each cost item into one of the business functions of the value chain, either (1) R&D, (2) design, (3) production, (4) marketing, (5) distribution, or (6) customer service.

Item:

a. cost of samples mailed to promote sales of a new product

b. labor cost of workers in the manufacturing plant

c. bonus paid to a person with a 90% satisfaction rating in handling customers with complaints

d. transportation costs for shipping products to retail outlets