Stronx Consulting tests its salespeople on listening skills and then implements a training program to teach all of them to be better listeners. At the conclusion of the last training session, it conducts a posttest to measure listening skills again. Based on the results, the training manager reports that the training program increased listening skills by a significant amount. Which of the following statements represents the main weakness in the manager's conclusion?
The actual impact of the training can only be evaluated by comparing the performance of employees who underwent training and those who did not.

Self-interest achieve society’s economic goals because producers know which goods consumers want the most. as consumers and producers exercise their freedom to act in their own self-interest, markets will produce the desired goods at the lowest possible cost. consumers and producers both operate based on society’s economic goals. consumers know which goods can be produced at the lowest cost. there is a wide variety of desired goods and services in a market system because producers determine what to produce. consumers change their minds frequently. there is always a need to produce something new and improved. individual wants are diverse. what is produced is ultimately determined by consumers, because if the goods offered are not what consumers want, consumers will not buy them. producers, because they are driven by profits. producers, because they determine what to produce. consumers, because they participate in marketing surveys.