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Let vec(alpha), vec(beta), vec(gamma) be...

Let `vec(alpha), vec(beta), vec(gamma)` be three unit vectors such that `vec(alpha).vec(beta)= vec(alpha),vec(gamma)=0` and the angle between `vec(beta) and vec(gamma)` is `30^(@)`. Then `vec(alpha)` is

A

`2(vec(beta ) xx vec(gamma))`

B

`-2(vec(beta) xx vec(gamma))`

C

`+- 2 (vec(beta) xx vec(gamma))`

D

`(vec(beta) xx vec(gamma))`

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