利用DTD给“公民”元素定义名称为“身份证号”的属性，该属性取值唯一且不可缺省，下面哪个选项（ ）正确？ A: <!ATTLIST 公民 身份证号 ID B: REQUIRED> C: <!ATTLIST 公民 身份证号 IDREF D: REQUIRED> E: <!ATTLIST 公民 身份证号 ID F: IMPLIED> G: <!ATTLIST 公民 身份证号 ID H: FIXED>

下面对DTD中属性声明中类型IDREF说法最准确的是

The electric furnace is used extensively to produce the high-grade steel _____ for the manufacture of steel alloys. A: to be required B: requiring C: required D: that required

To apply for the position, applicants _____ to create an online account. A: required B: are required

For profile controls, datums are: A: Allowable but not required B: Not allowed C: Required D: No answer

I am sure Mary has been here and （） A: did what required her B: has done what required her C: has done what was required of her D: did what has been required of her

The boss required ________.

As he ——to submit the according report before 4:30 p.m., the assistant hurried to Mr. Smith’s office. A: was required B. had required C. requires D. required

Active transport is characterized by ( ) A: Transport by carrier, no energy is required B: Through carrier transport, energy is required C: No carrier transport, no energy consumption D: Without carrier transport, energy is required

13.如果希望属性的取值唯一，则该属性应定义为 A: ID B: IDREF C: IDREFS D: ENTITY