Research Design/Analysis Question

A factory manager is concerned with employee turnover.  In a typical year around 85% of the employees who were working at the beginning of the year are still working in the factory at the end of the year.  Last year the rate was only 65%.  The manager implements a more flexible schedule for the employees in an effort to make them happy.  At the end of that year the retention rate for the factory workers is 79%, and the manager gets a pay raise for “solving the problem”.  

Below is a list of threats to validity discussed in chapter 14. Identify which of them is most likely to provide an alternative explanation for the results,

[removed]A) Attrition
[removed]B) History
[removed]C) Maturation
[removed]D) Regression
[removed]E) Retesting
[removed]F) Selection

Explain how this threat to validity could explain the results.