______ scientists have learned a lot about the universe, there is much we still don't know. ( 章节：1 难度：2)
- A、Once
- B、Since
- C、Though
- D、Unless
正确答案:C
答案解析:有
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______ scientists have learned a lot about the universe, there is much we still don't know. ( 章节：1 难度：2)
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