Why is serum IgM unable to activate complement prior to antigen binding?

A. Prior to antigen - binding, the complement binding sites on the Fc region of IgM are inaccessible.
B. Prior to antigen - binding, IgM exists as a pentamer but must be monomeric to activate complement.
C. Prior to antigen - binding, IgM is not associated with IgG which is required for complement activation.
D. Prior to antigen - binding, IgM is bound to a complement inhibitor that is released when the antibody binds antigen, activating complement.