Listen to the recording and answer the multiple-choice question by selecting the correct response. Only one response is correct.

Q. Which of the following best expresses the main idea of the passage?

(1) There are conflicting hypotheses about heterogeneity of the Earth’s mantle because few mantle elements have been thoroughly studied.
(2) Further research is needed to resolve the debate among geologists over the composition of mid-ocean ridge system.
(3) Current theories regarding structure of the Earth’s mantle cannot account for new discoveries regarding the composition of mantle xenoliths.
(4) There is clear-cut disagreement within geological community over the structure of the Earth’s mantle.

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