Of the Following Which Is an Example of a Prospective

Of the following which is an example of a prospective casual-comparative approach?

A) Katie was interested in whether or not differences in university persistence are found between students coming from large versus small high schools. She compared the percentage of students who had dropped out of college that had attended either a large or small high school.
B) Carrie compares college GPA of students who had either enrolled in high school AP classes and those who had not.
C) Molly compares the amount of time spent independently reading between those who were given phonics instruction in first grade and those who were given whole language.
D) Jason wondered if assigning vocabulary homework for his high school history class would help their performance on his final. He assigned one class weekly vocabulary homework and then compared their final exam scores to those of another class to which no weekly vocabulary homework was assigned.