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【单选题】
当材料的 孔隙率增大时,材料的( )降低。A.
密度
B.
密度
C.
憎水性
D.
抗冻性
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A.
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【单选题】设有关系 R 和 S ,在下列的关系运算中,( )运算不要求 R 和 S 具有相同的目数,也不要求对应属性的数据类型相同。
A.
R ∪ S
B.
R ∩ S
C.
R-S
D.
R × S