question: Q:An epidemiologist is interested in studying the clinical utility of a free computerized social skills training program for children with autism. A total of 125 participants with autism (mean age: 12 years) were recruited for the study and took part in weekly social skills training sessions for 3 months. Participants were recruited from support groups in a large Northeastern US city for parents with autistic children. Parents in the support group were very eager to volunteer for the study, and over 300 children were placed on a waiting list while the study was conducted. At baseline and at the end of the 3-month period, participants were observed during a videotaped social play exercise and scored on a social interaction rating scale by their parents. Social interaction rating scores following the 3-month intervention were more than twice as high as baseline scores (p < 0.001). During exit interviews, one parent commented, ""I knew from the start that this program was going to be life-changing for my son!"" This sentiment was echoed by a number of other parents. Which of the following is the most likely explanations for the study's result?"? 
  {'A': 'Social desirability bias', 'B': 'Recall bias', 'C': 'Observer bias', 'D': 'Sampling bias', 'E': 'Confounding bias'},