"what part of the antibody's structure determines its class? "

Out of the following given choices;
A) light (L) chain B) constant (C) regionC) variable (V) regionD) heavy (H) chain
The answer is B. This region is called the Fc region and composed of only heavy polypeptide chains only. It is used to classify antibody according to class. There are IgM, IgA,   IgG, IgE, and IgD. Fab is the varying region of the antibody that is significant in binding antigens.

When a virus infects a host cell, it uses the cell's machinery to make copies of its own genetic material in a process called replication. finally, during the process of self-assembly, that protein shell and the copies of the virus's genetic material are assembled into new viruses.