單項(xiàng)選擇題

給出下列命題: 1零向量的長(zhǎng)度為零,方向是任意的; 2如a,b都是單位向量,則a=b; 3向量AB與BA相等。則所有正確命題是（）

A.①
B.③
C.①③
D.①②

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