Norbert is conducting an experiment to investigate whether setting specific goals leads to higher commitment. To do this, he asks one group of participants (the experimental group) to think of some specific task-related goals. Meanwhile, he has another group of participants (the control group) do nothing. After this, he gives the participants in both the experimental and control group a work task. The experimental group completes a survey that asks them how committed they feel to the goals they set. The control group is given a survey with different questions, which asks them how committed they feel about completing the study. What threat to validity would prevent Norbert from comparing the survey results between these two groups? a. Regression to the mean b. A lack of generalizability (external validity) c. A testing effect d. An instrumentation effect