The angle between two forces of equal magnitude $R$,if the magnitude of their resultant is $\frac{R}{2}$,is:

  • A
    $\cos ^{-1}\left(-\frac{7}{8}\right)$
  • B
    $\cos ^{-1}\left(-\frac{5}{7}\right)$
  • C
    $\cos ^{-1}\left(-\frac{3}{7}\right)$
  • D
    $\cos ^{-1}\left(-\frac{3}{4}\right)$

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