The reason _______ he felt frustrated was ______ he lost his MP4.
Stop making noise, _______ the neighbors will start complaining.
Many students go to school _______ the tube in Beijing every day.
It was two weeks before the Spring Festival and the shopping centre was _____ with shoppers.
Leave your key with a neighbor _____you lock yourself out one day.
—Bob has gone to California. —Oh, can you tell me when he ______?