The problem of the present world is that those who are imaginative have little experience_________those who are experienced have weak imagination.

He had his camera ready ____he saw something that would make a good picture．  

_______ we got to the station, the train had left already.

I was just at the school gate    the bell rang.

I had been puzzled over the problem for over an hour without any result,      all at once a solution flashed across my mind.