Sam is the vendor test manager for an ecommerce portal enhancement project. He is in the process of identifying testing metrics that can be presented to the customer so as to highlight the effectiveness of the testing process. Identify which metric should be on top of his list.

A$)$ Defect classification by severity$/$ priority

B$)$ Total Defect Containment Effectiveness

C$)$ Valid Defect $\%$

D$)$ Requirement Stability Index