Psychology researchers observed that parents feel emotion while singing to their infants. The researchers hypothesized that this emotion noticeably affects the sound of the singing. To test this hypothesis the parents were recorded while singing to their infants and while singing with no infant present. They were instructed to make the two renditions as similar as possible. These recordings were then played for psychologists who had not observed the recordings being made. For 80 percent of the recordings, these psychologists were able to correctly identify, by listening alone, which recordings were of parents singing to their children. The researchers concluded that their hypothesis was correct. Which one of the following, if true, would most strengthen the researchers' reasoning? ['Most of the parents who participated in the study believed that the emotion they felt while singing to their infants affected their singing.', 'Parents displayed little emotion when singing with no child or adult present.', 'When a person feels emotion, that emotion provokes involuntary physiological responses that affect the vocal cords and lungs.', 'A separate study by the same researchers found that parents feel more emotion when singing to their own children than when singing to other children.']