If $K=\sin\large\frac{5\pi}{8}$$\sin \large\frac{5\pi}{18}$$\sin \large\frac{7\pi}{18}$,then the numerical value of $K$ is _____

Question

$\begin{array}{1 1}(A)\;\large\frac{1}{2}&(B)\;1\\(C)\;\large\frac{1}{8}&(D)\;\sqrt 2\end{array}$