One learns a knowledge by making mistakes and ______ them.

(2009年合肥市高三第一次教学质量检测)It was the training      he had as a young man     made him such a good engineer．

It was not until the entrance exams were in sight that      that I had so many books to cover and so many exercises to finish.

It ______ six years since they last saw each other.

With the development of science and technology，there is no doubt ________ Chinese astronauts will land on the moon in the near future.