Which of the following costs incurred by a paper manufacturer would be included in the group of costs referred to as conversion costs?

A) machine operator's wages.
B) sales person salaries.
C) lumber.
D) advertising costs.

Conversion costs are the costs incurred to convert raw materials into a finished product, and include direct labor and manufacturing overhead costs such as machine operator's wages. Sales person's salaries, lumber, and advertising costs are not considered conversion costs as they do not directly relate to the manufacturing process.