If $\alpha+\beta+\gamma=\pi$ then the value of $\sin^2\alpha+\sin^2\beta-\sin^2\gamma$ is equal to

Question

$\begin{array}{1 1}(a)\;2\sin\alpha &(b)\;2\sin\alpha\cos\beta\sin\gamma\\(c)\;2\sin\alpha\sin\beta\cos\gamma&(d)\;2\sin\alpha\sin\beta\sin\gamma\end{array}$