4. A group of physicians would like to examine which of the two most popular over-the- counter pain relievers (ibuprofen and aspirin) is most effective at reducing pain in adults over 40. The group finds 400 adults over 40 experiencing chronic pain: half of them are assigned a daily dose of ibuprofen and the remaining half a daily dose of aspirin. After several months the physicians find that there is not a significant difference in the amount of pain reduction reported from both groups. a. What is the explanatory variable? b. What is the response variable