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【单选题】
若向量 a ， b 满足| a |=1，| b |= 2 ，且 a ⊥ ( a + b ) ，则 a 与 b 的夹角为（ ）A.
π 2
B.
2π 3
C.
3π 4
D.
5π 6
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举一反三
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A.
Non degree
B.
Nonsense
C.
Non fiction
D.
non-governmental
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A.
the standard on which the judgment may be made is more important than the actual application of this judgment
B.
the function of law is important
C.
the study of ordinance is the most important
D.
practice is more important than criterion
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