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

a. Periodic physical inspection and reconciliation of plant and equipment to the detailed accounting records 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.