已知則      (        )

A. B.
C. D.的夾角為

C

解析試題分析:根據(jù)題意由于,則可知,而對于,從而說明向量成立,對于D,的夾角為,故錯誤,對于B,由于向量的坐標(biāo)不符合共線的公式,故錯誤,選C.
考點:向量的數(shù)量積
點評:主要是考查了向量的數(shù)量積的坐標(biāo)運算屬于基礎(chǔ)題

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