Eduardo's mother just bought a new vacuum cleaner that she loves. eduardo wants to know if it really cleans better than the old one. he gets a few identical carpet samples and works an equal amount of sand and dirt into each one.
then he gets a panel of ten people to rate how well each vacuum cleans.

which of the following experimental setups will be the most objective and have the least bias?

a. eduardo leaves 1/3 of the samples dirty. he divides the other samples into two groups. he cleans each group with a different vacuum. he puts the samples in a random order, then asks the panelists to rate how clean each sample is.

b. the panelists watch eduardo use each vacuum to clean a carpet sample, then they rate how clean each sample is.

c. each panelist cleans one carpet sample with each vacuum, then rates how clean each sample is.

d. eduardo leaves 1/3 of the samples dirty. he divides the other samples into two groups. he cleans each group with a different vacuum, then he asks the panelists to rate how clean each group is.