3. Commitment Devices: (a) Give an example of a costly commitment device that an individual could use. This can be an example discussed in class or any that you've seen. (b) Explain how the use of a costly commitment devices is inconsistent with a stan- dard assumption of classical economic theory. (c) Using the terminology of behavioral economics, explain what type of individual would use a costly commitment device. 4. Read the following description of a (fake) study, and answer a few questions below about its validity. Researchers wanted to study whether owning a larger refrigerator caused people to eat more calories. They visited college students' dorm rooms and measured the size of their refrigerators, and asked students to list all of the foods they had eaten that day. The researchers then calculated the number of calories consumed. They found that students with larger refrigerators ate more calories and concluded that a tax on refrigerator size would decrease calorie consumption and reduce rates of obesity in America. (a) In 1-2 sentences, describe one threat to the internal validity of this study. (b) In 1-2 sentences, describe one threat to the external validity of this study.