根據括號中的要求改寫下列句子。
1. He is going to Shanghai. (用現在完成時) ?
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2. I heard about the news about the space travel. (改為同義句) ?
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3. Scientists have found life on other planets.(改為否定句) ?
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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 stati
on. (用already) ?
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