Answer:
C. The note on the package only told customers to call in the event that they had a concern, which could bias the results negatively.
Step-by-step explanation:
Since customers were only told to call if they had concerns, all of the calls were about concerns or problems that they were having - meaning all of the calls were negative. However, they were never informed to call if they were happy or content with the product, meaning they were never informed of the people who had positive comments, only negative.