Although instinct enables organisms to make complex responses to stimuli, instinctual behavior involves no reasoning and requires far fewer nerve cells than does noninstinctual (also called flexible) behavior. A brain mechanism capable of flexible behavior must have a large number of neurons, and no insect brain has yet reached a size capable of providing a sufficiently large number of neurons. Which one of the following can be properly inferred from the statements above? ['All organisms with large brains are biologically equipped for flexible behavior.', 'Only organisms with brains of insect size or smaller engage in purely instinctual behavior.', 'Insect behavior is exclusively instinctual.', "All organisms with brains larger than insects' brains are capable of some measure of flexible behavior."]