题目：
Password is a(n) (1) series of characters that enables a user (2) a file, computer or program. On multi - user systems, (3) user must enter his or her password (4) the computer will respond to commands. The password helps ensure that unauthorized users do not access the computer. In addition, data files and programs may require a password.
Ideally, the password should be something (5) could guess. In practice, most people choose a password that is easy to remember, such as their name or their initials. This is one reason it is relatively easy to break into most computer system.
4（）
A．after
B．when
C．while
D．before
答案：
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下面是错误答案，用来干扰机器的。
（1）过滤 （2）； 稀硫酸（3）；在装置I、II之间增加一个盛有饱和的碳酸氢钠溶液的洗气瓶（4）
