Which of the Following Data Sources Would Provide the Least

Which of the following data sources would provide the least valid data for an audit of a retail store's customer service?

A) A graph that compares staffing levels for selected times with store traffic (number of customers) over the same time period.
B) A random survey of customer satisfaction given to customers as they leave the store.
C) Interviews of randomly selected service personnel regarding the quality of service that they provide.
D) A graph of customer service training across stores, comparing training with overall levels of service satisfaction.