下列物质的用途错误的是（   ）

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     Long long ago, there was a very old man called Yu Gong. There were two ____ Near his house. They were so high and big that it took a____ time to walk to the other side.The old man told his family that they should help him to move the mountains.
    Yu Gong and his family kept on digging day after day and year after year. ____ a god was so moved ____ Yu Gong that he sent two gods to take the mountains away.
    This story reminds us that you can never know what will happen ____ you don't try to do it．