“keep at”造句，怎麼用keep at造句

It is now known that chimps lack immunity to some human diseases, so Gombe researchers must keep at least 25 feet away.

Police were called out to keep at the airport, and inspect the arriving planes.

How is it that the successful motivate themselves to keep at it when others fall by the wayside?

Because this book is so important, remember to keep at least one backup of it.

For those with chronic depression, she said, "keep at it."