The corpus callosum -- the thick band of nerve fibers connecting the brain' s two hemispheres -- of a musician is on average larger than that of a nonmusician. The differences in the size of corpora callosa are particularly striking when adult musicians who began training around the age of seven are compared to adult nonmusicians. Ttherefore, musical training, particularly when it begins at a young age, causes certain anatomic brain changes. Which one of the following is an assumption on which the argument depends? ['Musical training late in life does not cause anatomic changes to the brain.', 'Adult nonmusicians did not participate in activities when they were children that would have stimulated any growth of the corpus callosum.', 'For any two musicians whose training began around the age of seven, their corpora callosa are approximately the same size.', 'The corpora callosa of musicians, before they started training, do not tend to be larger than those of nonmusicians of the same age.']