Paragraph 5 implies that a special explanation of Ir in the boundary clay is needed because A. the Ir in microscopic meteorites reaching Earth during the Cretaceous period would have been incorporated into Earth's core B. the Ir in the boundary clay was deposited much more than a million years ago C. the concentration of Ir in the boundary clay is higher than in microscopic meteorites D. the amount of Ir in the boundary clay is too great to have come from microscopic meteorites during the time the boundary clay was deposited