What potential selection problem could compromise the validity of the experiment's findings when comparing the salaries of individuals who chose to participate in the job training program with those who decided not to participate?$($a$)$ The difference in average salaries between the two groups may solely reflect the effectiveness of the job training program.$($b$)$ Individuals who choose to participate in the job training program might have different levels ofmotivation or prior skills compared to those who do not, affecting their salary outcomes independently of the program.$($e$)$ The job training program might be more accessible to individuals living closer to the training facilities, thereby influencing their decision to participate.$($d$)$ Participants in the job training program may demand higher salaries purely based on their expectation of the program's prestige, not its actual content.