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Q.) A tetrahedron has vertices O (0, 0, 0), A (1, 2, 1), B (2, 1, 3) and C (-1, 1, 2), then angle between faces OAB and ABC will be
[A]cos⁻¹ (17/31)
[B]30°
[C]90°
[D]cos⁻¹ (19/35)