An audit firm is testing controls within the purchasing cycle. In which of the following procedures would the firm most likely apply sampling techniques?

a. Risk assessment procedures performed to obtain an understanding of internal control in the purchasing cycle. 

b. Tests of automated application controls involving check amount limits when effective information technology general controls are present. 

c. Analyses of controls to determine the appropriate segregation of duties in the purchasing cycle. 

d. Testing of operating effectiveness of controls over authorization of purchase orders.