If substantial batch-level or product-level costs exist, then overhead allocation based on a measure of volume such as direct labor-hours alone:

a. must be used for external financial reporting since activity-based costing cannot be used for external reporting purposes
b. will systematically over cost high-volume products and under cost low-volume products
c. is a key aspect of the activity-based costing model
d. will systematically over cost low-volume products and under cost high-volume products