Choose the word or set of words that, when inserted in the sentence, best fits the meaning of the sentence as a whole.

　　Their conversation was unsettling, for the gravity of their topic contrasted so oddly with the ------- of their tone.

　　(A) uniqueness

　　(B) rapidity

　　(C) lightness

　　(D) precision

　　(E) reverence

　　Explanation

　　Only "lightness" fits the logic of the sentence as being contrary to the "gravity" of the topic. Listening to others discuss a grave (or serious) topic in a light-hearted tone would be unsettling and confusing.

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（编辑：马菲）