Police were called out to keep at the airport, and inspect the arriving planes.
For those with chronic depression, she said, "keep at it."
Because this book is so important, remember to keep at least one backup of it.
It is now known that chimps lack immunity to some human diseases, so Gombe researchers must keep at least 25 feet away.
How is it that the successful motivate themselves to keep at it when others fall by the wayside?
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