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【单选题】
不允许表在指定列上具有相同的值,但允许有空值,这种约束属于( )A.
DEFAULT约束
B.
UNIQUE约束
C.
PRIMARY KEY约束
D.
FOREIGN KEY约束
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反馈参考答案：
举一反三
【单选题】When the headmaster spoke out his plan, most of the students ____ against it.
A.
are
B.
were
C.
do
D.
did
【单选题】When the headmaster came into the hall , all the students _______ to welcome him .
A.
sat down
B.
went out
C.
stood up
D.
turned back
【简答题】调车信号机显示一个（）灯光，表示准许越过该信号机。
【单选题】When the headmaster spoke out his plan, the _____of the students in the class ____ against it.
A.
majority; were
B.
most; were
C.
most; did
D.
majority; did
【单选题】When the headmaster spoke out his plan, the _______of the students in the class________ against it .
A.
most；were
B.
most；did
C.
majority；were
D.
majority ；did
【单选题】在Word 2010的编辑状态,执行编辑选项卡中“复制”命令后()
A.
被选择的内容被复制到剪贴板
B.
被选择的内容被复制到插入点处
C.
插入点所在的段落内容被复制到剪贴板
D.
光标所在的段落内容被复制到剪贴板
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