Which choice most logically completes the text?

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Interested in how the color of dogs' irises affects human responses to dogs, Akitsugu Konno et al. showed images of 12 dogs' faces to human participants and asked them to rate the dogs' kindness and trustworthiness as well as the likelihood that they would interact with or keep the dogs. The researchers had previously adjusted the images so that each dog was presented in two versions, one with light irises and one with dark irises. They distributed the images such that no participant saw both the light and dark versions of the same dog. Konno et al. found that participants responded more positively to the latter. The study's design allowed the researchers to exclude the possibility that the results _______

may have been different if participants had been able to see the dogs' irises instead of only seeing images that included the dogs' irises.

reflected a preference on the part of participants for facial features that happened to coincide with dark irises in the dogs depicted in the images.

were a function of participants emphasizing the colors of the dogs' irises over the dogs' kindness and trustworthiness when reacting to the images.

could be explained by participants having preexisting positive feelings about dogs with dark irises or preexisting negative feelings about dogs with light irises.