Listed below are four functions: y$1=2$x$,$ y$2=0,$ y$3=3$x$2,$ y$4=3$ Determine which of these functions are solutions to the following differential equations. $($Note: there may be equations which have more than one solution.$)($a$)$ xy$=2$y $($b$)$ y$=2($c$)$ xy$=$ y$$