Amino Acid Sequence Analysis of All of the Peptides Found

Amino acid sequence analysis of all of the peptides found in a single IgG antibody would reveal unique peptide sequences totaling ~600-700 amino acids. Using this estimate, the predicted molecular weight of an antibody protein would be ~70-75 kDa. Yet, an intact antibody protein has a molecular weight of ~150 kDa. The explanation for this discrepancy is:

A) IgG antibodies have many more heavy amino acids in them than most other proteins.
B) Each IgG antibody is a complex of two identical light chains and two identical heavy chains.
C) IgG antibodies tend to aggregate together during purification, thereby distorting molecular weight estimates.
D) Each IgG antibody is a complex of four identical polypeptides.
E) IgG antibodies are produced as dimers of two identical IgG monomers.