Pentameric IgM is highly effective at activating the classical complement pathway and agglutinating antigens. Which structural feature of IgM accounts for its high avidity despite having relatively low affinity for individual antigen epitopes?

Options:

• A: Its ten antigen-binding sites held together by a J-chain, allowing multivalent binding.
• B: The presence of a secretory component that stabilizes its structure.
• C: The flexibility of its hinge region, which is longer than that of IgG.
• D: Its ability to cross the placenta and activate fetal complement proteins.

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