已知向量 i =(1,0), j =(0,1), a = i -2 j , b = i +λ j ,且 a 与 b 的夹角为锐角,则实数λ的取值范围( )
A.
(-∞,-2)∪(-2, 1 2 )
B.
(-∞, 1 2 )
C.
(-2, 1 2 )
D.
(-∞,-2)
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