Which of the following internal controls is most likely to justify a reduc- tion of control risk concerning plant and equipment acquisitions?

a. Periodic physical inspection of plant and equipment by the internal audit staff.

b. Comparison of current-year plant and equipment account balances with prior-year actual balances.

c. Review of pre-numbered purchase orders to detect unrecorded trade-ins.

d. Approval of periodic depreciation entries by a supervisor independent of the accounting department. p-654