a back-up copy的用法和样例：
- Many software companies make an express provision for the user to make one back up copy.
- Always have a back up original format copy of your image. Never alter the original. Believe me you will regret it.
- Back up a copy of the database encryption keys.
- Personnel were contacted on their portables and as a back up could be contacted by cell phone.
- He drought along a file of document to back up his claim.