Tracy and eric are timing how fast marbles roll down an inclined plane. they plan to do this 5 times, and then take the average time for their results. what would be best way to improve the accuracy of their data?

a. include marbles of three different colors in their investigation
b. include the average of 20 trials down the inclined plane.
c. use marbles of 10 different marbles
d. use toy cars instead of marbles