Let \( P Q \) and \( Q R \) be diagonals of adjacent faces of a rectangular box, with its centre...

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Let \( P Q \) and \( Q R \) be diagonals of adjacent faces of a rectangular box, with its centre at \( O \). If \( \angle Q O R \), \( \angle R O P \) and \( \angle P O Q \) are \( \theta, \phi \) and \( \Psi \) respectively then the value of \( \cos \theta+\cos \phi+\cos \Psi \) is :
\( \mathrm{P} \)
(a) \( -2 \)
(b) \( -\sqrt{3} \)
(c) \( -1 \)
(d) 0
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