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A high percentage of injuries to the anterior cruciate ligament (ACL) occurs in athletes. A sophisticated statistical analysis looked at the number of athletes who suffered an ACL injury, and assigned numerical values to both the nature of the stress that produced the injury and to the resultant level of damage to the ACL. Researchers expected that, controlling for level of injury-causing stress, all athletically inclined individuals would show similar tendency toward ACL injury regardless of the specific sport. A surprising outcome is that, equalizing for injuring-causing stress, it appears that gymnasts have a much higher tendency toward ACL injury than do members of the National Football League (NFL), but this is most likely because _______________

Which of the following most logically completes the argument?