According to the Text, What Is One of the Special

According to the text, what is one of the special challenges in measurement in evaluative research when you are evaluating the progress of a single client using a single-subject design?

A) You do not have any information in single-subject research about whether the intervention is the cause of the measured client growth rather than something else.
B) You will not know whether the measurement tools are reliable or valid.
C) The time it takes for the client to complete the measurement tool may be too intrusive into the service process.
D) You cannot draw conclusions from your study if you are measuring only one client.