How many of the following equations are written in function form?  ($1$) $2x-y=3x+1$;~($2$) $2x=-5y+2$;~($3$) $y=7x+12$;~($4$) $y+6=12x-17$;~($5$) $y=x+z+ab$ 
选项为:A:$$1$$  B:$$2$$  C:$$3$$  D:$$4$$  E:$$5$$  请回答其中一个选项.