If $\vec{A}=\vec{B}+\vec{C}$ and the magnitudes of $\vec{A}, \vec{B}$ and $\vec{C}$ are 5,4 and 3 units respectively, the angle between $\vec{A}$ and $\vec{C}$ is
(a) $\cos ^{-1}\left(\frac{3}{5}\right)$
(b) $\cos ^{-1}\left(\frac{4}{5}\right)$
(c) $\frac{\pi}{2}$
(d) $\sin ^{-1}\left(\frac{3}{4}\right)$