Does talking on a hands$-$free cell phone distract drivers? Researchers recruit $40$ student subjects for an experiment to investigate this question. They have a driving simulator equipped with a hands$-$free phone for use in the study. Each subject will complete two sessions in the simulator: one while talking on the hands$-$free phone and the other while just driving. The order of the two sessions for each subject will be determined at random. The route, driving conditions, and traffic flow will be the same in both sessions.

$($a$.)$ What type of design did the researchers use in their study?

$($b$.)$ Explain why the researchers chose this design instead of a completely randomized design.

$($c$.)$ Why is it important to randomly assign the order of the treatments?