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. altered primary structure.
b. altered secondary structure.
c. altered tertiary structure.
d. altered quaternary structure.

a. altered primary structure.

d. altered quaternary structure.

Explanation:

The primary structure of a protein represents the amino acid sequence of its polypeptide chains. Change in the single amino acid in the beta chain of hemoglobin represents the changed primary structure of this polypeptide chain. This altered primary structure also changes the quaternary structure of the protein.

The quaternary structure represents an arrangement of two or more polypeptide subunits of a protein. Substitution of a negatively charged amino acid (glutamic acid) for valine (uncharged and nonpolar amino acid) creates a hydrophobic sticky point at the surface of beta chains of hemoglobin and facilitates their aggregation.

the answer is:

carbon dioxide