Recent research shows that sound change (pronunciation shift) in a language is not gradual. New sounds often emerge suddenly. This confounds the classical account of sound change, whose central tenet is gradualness. Since this classical account must be discarded, sound-change theory in general must also be. Which one of the following, if assumed, does most to justify the argument's conclusion? ['The emergence of new sounds appears to be random.', 'All theories of sound change rely heavily on the classical theory.', 'For most languages, historical records of their earlier stages are scarce or nonexistent.', 'The data on which the classical account of sound-change theory was based are now known to be inaccurate.']