This is so difficult a problem that the scientist is known to ______ on it for a

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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

解析这个科学家一直对如此难的问题研究了大约十年，但是仍然没有找到答案。不定式to have been working...作主语scientist的补足语，要用完成进行式表示。

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