单选题 Password is a secret series of (69) that enables a user to access a file, computer, or program. On multi-user systems, each user must enter his or her password before the computer will respond to (70) . The password helps ensure that (71) users do not access the computer. In addition, data files and programs may require a password.

【正确答案】 D
【答案解析】本题意思为“密码是一组字符串”，characteristic此处意为“字符串”。答案为D。
【正确答案】 B
【答案解析】题意为“计算机响应命令”。instruction意思为“指令”。command意思为“命令”。答案为B。
【正确答案】 D
【答案解析】unauthorized意思为“未经过授权的”。结合题意，答案为D。