In a manufacturing firm, allocation of indirect costs based on direct labor-hours or machine hours:

A. Is a key aspect of the activity-based costing model.

B. Will systematically over-cost high-volume products and under-cost low-volume products.

C. Will systematically over-cost low-volume products and under-cost high-volume products.

D. Must be used for external financial reporting.

E. None of the above.

Why is it B and what is volume bias?