中国大学MOOC: They have been waiting for many hours to see the singer, but the airplane must have been ____ .
举一反三
- 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
- 中国大学MOOC: Winning first place must have been quite a __________.
- The room is in a terrible mess; it （）cleaned. A: can’t have been B: shouldn’t have been C: mustn’t have been D: will have been waiting
- 中国大学MOOC: Many derivatives have been modified by partial synthesis for ________ use.