Aresearcher wondered whether drivers treat bicycle riders differently when they wear helmets. he rigged his bicycle with an ultrasonic sensor that could measure how close each car was that passed him. he then rode on alternating days with and without a helmet. of the 30003000 times that a car passed him, he found that when he wore his helmet, motorists passed 3.273.27 inches closer to him, on average, than when his head was bare. what is the whowho in this study? that is, identify the cases under study.
a. the bike riders
b. each instance of a car passing a rider
c. bike helmets
d. the cars