A \( \cos^{-1}\left(\frac{8\sqrt{21}}{63}\right) \)
B \( \cos^{-1}\left(\frac{\sqrt{21}}{63}\right) \)
C \( \cos^{-1}\left(\frac{5\sqrt{21}}{63}\right) \)
D \( \sin^{-1}\left(\frac{8\sqrt{21}}{63}\right) \)
Correct Answer: Option A
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