Imagine you are collaborating with researchers studying the effect of marijuana on driving. Participants are assigned by chance to either a) receive a pill that contains THC, the active ingredient in marijuana, OR b) a harmless placebo pill. Neither the researchers nor the participants know who is getting which pill. After taking the pill, participants are asked to drive a computerized driving simulator. The number of collisions participants have is measured.This study uses a(n) _______________ design.