Of the following, which is an example of a retrospective causal--comparative approach?

a. Allison believed allowing students open campus for lunch would promote better school attendance so she allowed one high school in the district to have open campus while the maintained their closed campus. She then assessed differences in attendance at the end of the semester.

b. Ben was interested in the benefits of outdoor recess on classroom behavior. One-class he had take year---round outdoor recess and the other he scheduled for indoor recess in the gym. At the end of the school year he examined referrals for classroom management for both classes.

c. James hypothesized that adults who were scouts as children would possess more sophisticated environmental attitudes. He compared the environmental attitudes of adults who were scouts to those of adults who were not scouts.

d. Lisa was curious whether after-school snacks promoted healthier dinner eating habits so she compared children from one day care setting that provided after school snacks to one that did not. At the end of the semester she had parents rate dinner eating behaviors for both groups of children.