23. _______ the plane have taken off ontime— I’m afraid not. The fog was too thickthen.

I had meant to meet you at the airport this morningbut I was too busy for I_______ a report

The children enjoyed ______ in the zoo.

Dr. Black is a professor of chemistry._______, he is a famous writer.

（2014山东省实验中学高三第二次模拟考试）7．Perhaps it will never be known __________ he did with those materials on the incident．