Gotera: Infants lack the motor ability required to voluntarily produce particular sounds, but produce various babbling sounds randomly. Most children are several years old before they can voluntarily produce most of the vowel and consonant sounds of their language. We can conclude that speech acquisition is entirely a motor control process rather than a process that is abstract or mental. Which one of the following is an assumption required by Gotera's argument? ['Control of tongue and mouth movements requires a sophisticated level of mental development.', "Speech acquisition is a function only of one's ability to produce the sounds of spoken language.", 'The initial babbling stage is completed during infancy.', 'The initial babbling stage is the first stage of the speech acquisition process.']