Normal hemoglobin is a tetramer, consisting of two molecules of β hemoglobin and two molecules of α hemoglobin. In sickle-cell disease, as a result of a single amino acid change, the mutant hemoglobin tetramers associate with each other and assemble into large fibers. Based on this information alone, we can conclude that sickle-cell hemoglobin exhibits .A) only altered primary structureB) only altered tertiary structureC) only altered quaternary structureD) altered primary structure and altered quaternary structure; the secondary and tertiary structures may or may not be altered