In computing, passwords are commonly used to limit ______to official users. Yet the widespread use of passwords has serious problems. Office workers now have to remember an _______of twelve system passwords. In theory they should use different passwords for each site, but______ these would be impossible to remember. So many people use the same password for all. An additional problem is that the majority use simple words such as hello, or names of family members, instead of more secure_______of numbers and letters, such as 6ANV76Y. This permits computer hackers(黑客)to download dictionaries, which will quickly find the word and allow them access. Therefore, it is clear that the idea of passwords, which have been used as _____devices for thousands of years, may need rethinking.