One way to measure self-discipline is the $1-now or $2-later test (which the researchers call the delay choice task) in which we see if a subject would decline to accept a dollar bill now when told they could wait a week and get two dollars. highly self-disciplined adolescents outperformed their more impulsive peers on every academic-performance variable, including report card grades. which of the following is correct?

a. this study is an experiment, since it was based on a random sample.
b. this study is an observation study since the researchers did not assign the students to one of the self-discipline categories.
c. this study is an experiment, since each subject was classified into a self-discipline category ($1-now or $2-later) and to report card grades.
d. this study is an experiment, since it was based on a random sample