A Researcher Is Interested in Whether Chewing Gum During Tests

A researcher is interested in whether chewing gum during tests helps students to perform better on those tests. Which one of the following scenarios is the best way to do it?

A) Using two existing classes, randomly choose which class will chew gum during tests.
B) Using the same group of students, randomly assign half of the school weeks to include gum chewing on all tests during those weeks.
C) Using 70 students, randomly assign them to two classes, with one of these two classes to use chewing gum during tests.
D) Using twenty existing classes at several local schools, randomly assign half of the classes to chew gum during tests.