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life began on earth is one of the biggest puzzles that scientist found hard . A.

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life began on earth is one of the biggest puzzles that scientist found hard . A.

       ______ life began on earth is one of the biggest puzzles that scientist found hard ______.

A. How; solving                                 B. When; to be solved

C. How; to solve                                D. Which; to solve

试题答案

C

标签：theofbiggestthatpuzzles

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