Read the statement from a paper comparing Keats's poem "Ode on a Grecian Urn" and "On Seeing the Elgin Marbles."While the message in "Ode on a Grecian Urn" is that art is an eternal and unchanging truth, the message in "On Seeing the Elgin Marbles" is that art changes over time.

The statement compares the poems’ central messages: one presents art as eternal and unchanging, while the other presents art as changing over time. A poem’s central message or insight is its theme, so the correct choice is “themes.”

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