Identify the Threat to Internal Validity in Each of the Following

Identify the threat to internal validity in each of the following:
a.Some researchers who were given rats to train were told incorrectly that the rats had been bred for intelligence; other researchers were told that their rats were especially dull.The experimenters who had the "smart" rats reported faster learning than did the experimenters with the "dull" rats.
b.After the first exam, a professor divided his statistics class into high performers and low performers.The low performers were invited to attend special tutoring sessions where the professor not only reviewed lecture material but also discussed how to take notes and how to study for a statistics test.On the second test, the professor found that the students who received the tutoring improved considerably.The professor concluded that the tutoring sessions were effective in improving the test performance of those students.
c.A researcher divided people attending a weight control clinic into two groups.Both groups were given the same diet to follow, but the experimental group was also asked to keep a record of everything they ate, to weigh all the food in grams, and to count the calories.At the end of the study, the researcher found that the experimental group had lost more weight.The researcher concluded that the experimental treatment was more effective than diet alone.