If "All primate brains may have frontal cortex" is true,. Which of the following must be true? ? ['Not all primate brains have a frontal cortex, which is not necessary.', 'All primate brains have a frontal cortex, which is inevitable.', 'All primate brains have a frontal cortex.', 'Not all primate brains have the frontal cortex, which is possible.']