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[单选题]
已知向量a=(2，-4)，b=(1，2)，c=(1，-2)，d=(-2，-4)，则其中共线的有（ ）
A
a与d共线，b与c共线
B
n与b共线，c与d共线
C
n与c共线，b与d共线
D
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参考答案
试题答案：
C
答案解释：
参考解析：由于向量a=(2，-4)，c=(1，-2)，有2×(-2)-(-4)×1=0，所以a与c共线．又由于向量b=(1，2)，d=(-2，-4)，有1×(4)-2×(-2)=0，所以b与d也共线．故选C．
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