Which of the following statements about a process costing system is incorrect?

a. In a process costing system, each processing department has a work in process account.

b In a process costing system, equivalent units are separately computed for materials and for conversion costs.

c In a process costing system, overhead can be underallocated or overallocated just as in the job-order costing.

d. In a process costing system, material costs are traced to units of products.