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【单选题】
设有关系R(A,B,C.,与SQL语句:SELECT DISTINCT A FROM R WHERE B=17等价的关系代数表是()。A.
ΠA(R)
B.
σB=17(R)
C.
ΠA(σB=17(R))
D.
σB=17（ΠA(R)）
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举一反三
【单选题】He had two houses. He lived with his wife and child in the outer house, ______ was comfortable and clean.
A.
which
B.
that
C.
it
D.
where
【单选题】I am hunting for a house: nice, bright, comfort table and ________ with a big garden.
A.
all over
B.
after all
C.
above
D.
in all
【简答题】We found the sofas of the coffee house(comfort)____ and fashionable.
【简答题】单缝的宽度b=0.40mm，以波长λ=589nm的单色光垂直照射，设透镜的焦距f=1.0m．求：
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