(WILL MARK BRAINLIEST) A grocery-store owner wonders whether the width of the checkout aisles influences how fast the checkout lanes move. Which of these would be the best test of this idea?

a) removing cluttered merchandise racks from the checkout aisles
b) opening new aisles whenever there are more than three people in line
c) encouraging checkout clerks to talk with customers
d) posting flyers for sales in the checkout aisles
e) all of these