Sometimes when their trainer gives the hand signal for "Do something creative together, " two dolphins circle a pool in tandem and then leap through the air simultaneously. On other occasions the same signal elicits synchronized backward swims or tail-waving. These behaviors are not simply learned responses to a given stimulus. Rather, dolphins are capable of higher cognitive functions that may include the use of language and forethought. Which one of the following, if true, most strengthens the argument? ['The dolphins often exhibit complex new responses to the hand signal.', 'Some of the behaviors mentioned are exhibited by dolphins in their natural habitat.', 'Dolphins do not interact with humans the way they interact with one another.', 'The dolphins are given food incentives as part of their training.']