A child learning to play the piano will not succeed unless the child has an instrument at home on which to practice. However, good-quality pianos, whether new or secondhand, are costly. Buying one is justified only if the child has the necessary talent and perseverance, which is precisely what one cannot know in advance. Consequently, parents should buy an inexpensive secondhand instrument at first and upgrade if and when the child' s ability and inclination are proven. Which of the following, if true, casts the most serious doubt on the course of action recommended for parents? ['Learners, particularly those with genuine musical talent, are apt to lose interest in the instrument if they have to play on a piano that fails to produce a pleasing sound.', 'Very young beginners often make remarkable progress at playing the piano at first, but then appear to stand still for a considerable period of time.', 'Reputable piano teachers do not accept children as pupils unless they know that the children can practice on a piano at home.', 'In some parents, spending increasing amounts of money on having their children learn to play the piano produces increasing anxiety to hear immediate results.']