Which one of the following statements is incorrect?
a. Manufacturing overhead includes all direct material and direct labor costs.
b. Indirect materials include those materials that become part of the product but cannot be traced to specific products.
c. Direct labor includes the wages paid to factory workers who do the actual assembly of a product.
d. Direct materials include those materials that become part of the product and can be traced to specific products.
e. Indirect labor includes the salaries of manufacturing supervisors.