You __________ to leave until you __________ your work.
Since Tom ______ downloaded a virus into his computer, he can not open the file now.
At minus 130℃, a living cell can be ______ for a thousand years.
The traditional view is ______ we sleep because our brain is “programmed” to make us do so.
______ he referred to in his article was unknown to the general reader.
Small sailboats can easily turn over in the water ______ they are not managed carefully.