Dr. Anguissola conducted the following experiment: Following IRB approval of her study, she recruited parents of forty 1-year-old children. She divided the families randomly into two groups of 20 and gave the infants in Group A educational CDs, while Group B received classical music CDs. Parents were instructed to play the CDs they received to their infants each night. After six months, Dr. Anguissola measured cognitive skills of the infants in both groups using a standardized age- appropriate assessment, in order to determine if one type of CD boosted cognitive development compared to the other type of CD. The following two questions are about this study. What is the independent variable in Dr. Anguissola's study?