2. 外语
  3. This is so difficult a problem that the scientist is known to______ on it for about ten years, but hasn’t found an answer yet.

This is so difficult a problem that the scientist is known to______ on it for about ten years, but hasn’t found an answer yet.

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问题 This is so difficult a problem that the scientist is known to______ on it for about ten years, but hasn’t found an answer yet.
选项 A、have worked
B、have been working
C、be working
D、work on
答案B
解析 不定式to have been working…作主语scientist的补足语，用完成进行式表示：这个科学家已对如此复杂的问题研究了大约10年，并且还要继续研究下去。
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