Of the Following Which Is an Example of a Prospective

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

A) Katie was interested in whether 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 compared college GPA of students who had either enrolled in high school AP classes and those who had not.
C) Molly compared 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 investigated whether assigning vocabulary homework for a high school history class would help their performance on his final. He identified one class that had weekly vocabulary homework and then compared their final exam scores to those of another class to which no weekly vocabulary homework was assigned.