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【多选题】
下列选项中哪几个属于关系模型的特点（）。A.
每一列不可再分
B.
同一个关系中不能出现相同的属性名
C.
关系中不允许有完全相同的元组
D.
关系中任意交换两行的位置不影响数据的实际含义
E.
关系中不允许任意交换两列的位置
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反馈参考答案：
举一反三
【单选题】I'm sure he hasn't sent the letter. If he ______ it, I ______ .
A.
sent, got
B.
sent, have got
C.
sent, would go
D.
had sent, would have got
【多选题】病毒的微生物学特征有（）。
A.
只含有一种核酸，DNA或RNA，为单股或双股
B.
严格的细胞内寄生，没有完整的酶系统，不能独立的进行生长和繁殖
C.
对一般的抗生素和作用于微生物代谢途径的药物敏感
D.
没有核糖体和转运RNA，无合成蛋白质所必须的酶
【单选题】Wre sent a telegram yesterday as we______any letter from him for a long time.
A.
haven't received
B.
didn't receive
C.
hadn't received
D.
would not receive
【单选题】What did Goodyear do as the first step to fight against absenteeism?
A.
They sent a letter to each employee's home through the post office.
B.
They sent letters to employees' homes in person.
C.
They gave away prizes first.
D.
They punished its employees severely.
【简答题】What was sent to the receiver of the letter?
【简答题】He sent me _____ letter, but _____ letter didn't arrive.
【单选题】I' m sure the letter won' t arrive in time ______ it is sent by air.
A.
if
B.
until
C.
because
D.
unless
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