In our online life, we need to use passwords frequently. We use passwords_____（21）e-mail, gaming sites, social networking sites, and other shopping sites._____（22）, the passwords most people use are not very_____（23）and can easily be"broken"by others. In fact, the most commonly used passwords are so simple that it requires very_____（24）effort to figure them out. Can you guess_____（25）the most commonly used passwords are?They are：Names of baseball teams, birth dates of a family_____（26）, the year of a special sports event, the random numbers like 156468, _____（27）the name of a friend, pet, favorite TV star, or band.
There are programs_____（28）to break into people's online accounts. These programs are_____（29）of trying every word in the English dictionary and the dictionaries_____（30）many foreign languages, in their effort to break into an account._____（31）can even search words backward. Some will try_____（32）words or words that are followed by numbers, _____（33）school222. These programs can test millions of passwords in a few minutes. So, you are advised to be careful about_____（34）passwords so that they will be hard to break. You are also advised not to make them_____（35）hard to remember. Meanwhile, you need to change them once in a while.
21._____
A.to B.with C.by D.for正确答案D
