Pit vipers are so called because of the small pits on the sides of their heads which function as infrared sensors, giving the viper a thermal impression of objects in its vicinity. Because all species of pit vipers are predators, scientists have long thought that these sensors were used to find prey. One researcher has hypothesized, however, that the infrared sensor serves primarily to assist the viper in judging the size of approaching predators and how far away they are. Which one of the following, if true, would most support the researcher's hypothesis? ['Pit vipers have a well-developed sense of smell that they use to supplement the thermal impressions they receive from their infrared sensors.', 'Pit vipers do not differ in their predatory behavior from closely related vipers without pits, but they do differ markedly from these other species in their strategies of defense against predators.', 'The rattle of the diamondback rattlesnake, one of the most common kinds of pit viper, functions as a defense mechanism to intimidate predators.', 'Pit vipers are distinguishable from other viper species not only by their pits but also by the chemical composition of their venom.']