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【单选题】
单相半波电阻负载可控整流电路,直流输出电压平均值 U d 是( )。A.
（ A）0.45U 2
B.
（ B）0.45U 2 cosα
C.
（ C）0.9U 2 cosα
D.
（ D）0.9U 2
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A.
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D.
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A.
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B.
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A.
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B.
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C.
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