When exercising the muscles in one' s back, it is important, in order to maintain a healthy back, to exercise the muscles on opposite sides of the spine equally. After all, balanced muscle development is needed to maintain a healthy back, since the muscles on opposite sides of the spine must pull equally in opposing directions to keep the back in proper alignment and protect the spine. Which one of the following is an assumption required by the argument? ['One should exercise daily to ensure that the muscles on opposite sides of the spine keep the back in proper alignment.', 'Muscles on opposite sides of the spine that are equally well developed will be enough to keep the back in proper alignment.', "If the muscles on opposite sides of the spine are exercised unequally, one's back will be irreparably damaged.", 'Exercising the muscles on opposite sides of the spine unequally tends to lead to unbalanced muscle development.']