Which of the following is an example of selection bias (Select only one option): a) A study compared the academic performance of students in a school that offers a new tutoring program to students in a nearby school that does not offer the program. The students in the two schools are not randomly assigned to the treatment and control groups. b) A study uses a randomized controlled trial to evaluate the effectiveness of a new parenting program. However, some parents drop out of the study before the end of the program, which could affect the studys findings. c) A study uses a pretest-posttest design to evaluate the impact of a new mental health intervention. However, the participants in the study were all recruited from the same clinic. d) A study evaluates the effectiveness of a new fitness program by comparing the fitness levels of participants before and after the program. However, the participants in the program are all volunteers who are motivated to improve their fitness.