D Question 3 1 pts A study conducted by researchers from the Department of Education wanted to know the average debt of college students in the United States. In order to obtain a sample representative of all students, the researchers divided college students into the four classes (freshman, sophomore, junior, and senior) and then took a random sample of students from each class. Which sampling method did they use? O Stratified random sampling O Systematic random sampling O Cluster sampling O Simple random sampling