Why Is the Postpurchase Evaluation of a Vendor and the Product

Why is the postpurchase evaluation of a vendor and the product necessary? 

A)  To determine the dues that are payable to the vendor
B)  To compare the purchases of a firm with those made by competitors
C)  To determine whether the vendor and the product is acceptable for future purchases
D)  To know the strategic plan and the profits made by the company in a year's time