It is many years ____ we enjoyed ourselves on the great occasion. 

_____ you are in a speech contest or you are giving a talk in class, a little preparation will make your presentation organized and dynamic.

Qian Xuesen tried many times _________ he found the solution to the puzzle about space technology.

 —What should we pay more attention to if we go north in winter, Mum?—Nothing much. Take warm clothes________ the weather is cold.

This college allows students to develop their own interests, to ______　extent they wished．