Which statement correctly distinguishes a primary immune response from a secondary immune response, and which is more extensive and prolonged?

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The key idea is how the immune system uses memory to change the response upon re-exposure. On the first encounter with an antigen, naive B cells must be activated, proliferate, and differentiate into plasma cells, producing antibodies (IgM first, then IgG) in a response that builds slowly and is moderate in magnitude. When the same antigen appears again, memory B cells respond quickly and robustly, generating a much larger amount of high-affinity antibodies and keeping antibody levels elevated for a longer period due to long-lived plasma cells. This makes the secondary response more extended and sustained.

So the statement that the primary response is the initial response and the secondary response becomes more prolonged when the antigen reappears aligns with how memory accelerates and amplifies the reaction.