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.

admin2016-02-04  74
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。
转载请注明原文地址:https://kaotiyun.com/show/UTPO777K
0

在职攻硕英语联考

相关试题推荐
随机试题
最新回复(0)