The Problem with Concluding That the Cause of Stuttering Has

The problem with concluding that the cause of stuttering has been found by research revealing a particular characteristic in adults who stutter that was not present in non-stuttering controls is that:

A) the characteristic could be typical of groups with other communication disorders not tested as controls
B) the characteristic could emerge at a later age, but not be present in children at stuttering onset
C) the characteristic and the stuttering might both have been caused by another variable
D) all of the above