He is a man of great help, with ________ help you can solve the problem easily.
Fred entered without knocking and, very out of breath, sank _________ a chair.
The open-air celebration has been put off __________ the bad weather.
__________ fire, all exits must be kept clear.
Some students often listen to music __________ classes to refresh themselves.
— __________ when has the country been open to international trade?— 1978, I suppose.