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【单选题】
已知关系R有20个元组，S有10个元组，则R∪S和R∩S的元组数不可能是A.
25、5
B.
28、2
C.
30、10
D.
20、10
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反馈参考答案：
举一反三
【简答题】It
costs four times as much to gain a new as it costs to keep an old one.
【单选题】I like this 3G cellphone very much, but it costs almost _______that one.
A.
four times as many as
B.
as four times many as
C.
four times as much as
D.
as four times much as
【单选题】A commonly used rule of thumb is that it costs approximately ____ times as much to get a new customer as it does to keep an existing customer.
A.
three
B.
four
C.
five
D.
six
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