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Let vec a . vec b be two unit vectors an...

Let `vec a . vec b` be two unit vectors and `theta` is the angle between them. Then `vec a + vec b` is a unit vector if

A

` theta = pi/4`

B

`theta = pi/3`

C

`theta = pi/2`

D

`theta = (2pi)/3`

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The correct Answer is:
D
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