George is planning an experiment to see how fast different sized cans of soup can roll down a ramp in his kitchen. He will be rolling them down a ramp and them time how fast it take them to travel 1 meter after the ramp. George writes the following procedure to conduct his experiment. Gather three different sized soup cans, a 30 cm ramp, a timer, a meterstick, and a scale.
Weigh the first can and record the size
Put the meterstick at the end of the ramp
Roll the soup can down the ramp, time how long it takes to move along the meterstick
Calculate the velocity
Repeat steps 2-5 for the other cans.
Which of the following should George add in order to improve his procedure?

a
Record the velocity of every can
b
Find different objects other than cans to work with
c
Change the Height of the ramp to make the cans go faster
d
Mention that he needs to clean up his materials