Cho \(\cos a = \frac{3}{4},\,\,\sin a > 0,\,\,\sin b = \frac{3}{5},\,\,\cos b...

A. \(\frac{3}{5}\left( {1 + \frac{{\sqrt 7 }}{4}} \right).\)

B. \(\frac{3}{5}\left( {1 - \frac{{\sqrt 7 }}{4}} \right).\)

C. \(-\frac{3}{5}\left( {1 - \frac{{\sqrt 7 }}{4}} \right).\)

D. \(-\frac{3}{5}\left( {1 + \frac{{\sqrt 7 }}{4}} \right).\)