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Use "back-up" in a sentence
"And try to have some sort of long-term back-up plan."
"The new site lacked sufficient customer call back-up, and when Ad Age reported that customers were spending 19% more on the site, the company cheered, only to learn that really meant people were spending that time in frustration."
"Two girls are threatening each other with knives, encouraged by a gang of fellow-teenagers: before police back-up arrives, Lynn successfully deals with the immediate threat, until someone in the crowd fires a gun - Lynn is injured and one of the girls is killed."