The article "Study Points to Benefits of Knee Replacement Surgery Over Therapy Alone" + describes a study to compare two treatments for people with knee pain. In the study, 50 people with arthritis received knee replacement surgery followed by a program of exercise. Another 50 people with arthritis did not have surgery but received the same program of exercise. After 1 year, 85% of the people who had surgery and 68% of the people who did not have surgery reported pain relief. (a) Why is it important to determine if the researchers randomly assigned the subjects to one of the two groups? O If the researchers randomly assign the subjects to one of the two groups, then the experiment will always be double-blind. Allowing subject participants to choose which group they want to be in could introduce systematic differences between the two experimental conditions (knee replacement surgery with exercise and exercise therapy alone), resulting in potential confounding. If the researchers randomly assign the subjects to one of the two groups, then the experiment will always be single-blind. Allowing subject participants to choose which group they want to be in could introduce systematic differences between the two response variables (knee replacement surgery with exercise and exercise therapy alone), resulting in potential confounding. Researchers must randomly assign the subjects to one of the two groups in order to ensure that there is a confounding variable. (b) Explain why you think that the researchers might have wanted to include a control group in this study. (Enter your free-form answer using as many characters as necessary.)