Prove that the following wffs are valid arguments.

$($a$)(3$ points$)($x$)($N$($x$))($x$)($G$($x$)->$ N$($x$))($x$)($G$($x$)$ C$($x$))->($x$)$C$($x$)$

$($b$)(3$ points$)($x$)($y$)($A$($x$)$S$($x$,$ y$)->$ D$($y$))($x$)($y$)($A$($x$)$S$($x$,$ y$))->($x$)$D$($x$)$