The Most Effective Procedure to Verify Compliance with a Requirement

The most effective procedure to verify compliance with a requirement that materials be purchased from the lowest-priced source is to compare:

A) Prices paid for selected materials with prices listed on related purchase orders.
B) Bids obtained for selected purchases with related purchase orders.
C) Vendors' current prices with prices listed on related purchase orders.
D) Approved vendor lists with bids obtained for selected purchases.