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.
A: have worked
B: have been working
C: be working
D: work on
A: have worked
B: have been working
C: be working
D: work on
举一反三
- Mr. Green began to work in ABC Company in 1990. He ______ for ten years by next summer. A: will work B: will have worked C: has been working D: will have been worked
- We（）on it for many hours but we have not yet reached any concinsion. A: work B: have been worked C: have been working D: are working
- We ______ on it for many hours, but we have not yet reached any conclusion. A: are being worked B: are working C: have been working D: have been worked
- 4. We ________ on it for several hours but we have not yet reached any conclusion. A: A. work B: B. are working C: C. have been working D: D. have been worked
- I____(work) as an assistant manager for four years. A: work B: have worked C: am working D: have been working