A Trainer at the Gym Notices That His Clients Are

Multiple Choice

A trainer at the gym notices that his clients are more likely to work out regularly and not skip days at the gym, compared to people who don't use a personal trainer. He has also noticed that people who have a trainer are less likely to get injured. The trainer believes that he helps people to stay motivated and accountable, and that's why they go to the gym more regularly. What is the confounding variable in this example?

A) injuries
B) the trainer
C) time at the gym
D) motivation