In computing, passwords are commonly used to limit access to official users. Yet the wides

pread use of passwords has serious drawbacks. Office workers now have to remember an average of twelve system passwords. In theory they should use different passwords for each site, but in reality 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 combinations of numbers and letters, such as 6ANV76Y. This permits computer hackers to download dictionaries and quickly find the word that allows them access.

When system users forget their passwords there is extra expense in supplying new ones, while if people are forced to change passwords frequently they often write them down, making systems even less secure. Therefore, it is clear that the idea of passwords, which have been used as security devices for thousands of years, may need rethinking.

One possible alternative has been developed by the American firm Real User, and is called "passfaces". In order to access the system a worker has to select a series of photographs of faces from a randomly (随机地) generated sequence. If the pictures are selected in the correct order, access is granted. This concept depends on the human ability to recognize and remember a huge number of different faces, and the advantage is that such a sequence cannot be told to anyone or written down, so is more secure. It is claimed that the picture sequence, which used photographs of university students, is easier to remember than passwords, and it has now been adopted for the United States Senate.

What is the disadvantage of passwords as mentioned in Paragraph 1？

A．They do not ensure security.

B．They are difficult to remember.

C．They have to be changed frequently.

D．They limit computer accessibility.