A large lending company held career advancement training courses for lower-level employees. These courses were run on a volunteer basis and took place at night on the employee's own time. In evaluating the effectiveness of this course five years later, it was found that those persons who had taken the course had advanced further in the company than those who had not taken the course. What can be best concluded in this situation? a. The results may be due to the time when courses are given. b. The results may be due to differences in the motivation of the two comparison groups. c. The results may be due to the interval of evaluation. d. The results may be due to how the courses are facilitated