根据括号中的要求改写下列句子。
1. He is going to Shanghai. (用现在完成时) ?
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2. I heard about the news about the space travel. (改为同义句) ?
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3. Scientists have found life on other planets.(改为否定句) ?
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4. They haven't been to Mars. (用yet) ?
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5. The spacecraft has reached the moon. (用just)?
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6. Many astronauts have visited the space station. (用already) ?
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