A Professor Invites Students Who Did Poorly on the First

Multiple Choice

A professor invites students who did poorly on the first test to attend a special review session before the second test.Most do better on the second test.What conclusion,if any,can we draw,and why?​

A) We conclude that the study session helped, because the scores improved.
B) ​We conclude that the most motivated students attended the review session, because scores improved.
C) ​We draw no conclusion, because the study has too many independent variables.
D) ​We draw no conclusion, because the study lacks a control group.