How life began on earth is one of the biggest puzzles that scientists found hard . A.to solve——青夏教育精英家教网—

摘要：How life began on earth is one of the biggest puzzles that scientists found hard . A.to solve B.to be solved C.solved D. be solving

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

A. to solve B. to be solved C. solved D. be solving

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