$(9)$ Which functions have level curves that are parallel lines? Select all that apply.

$($a$)f(x,y)=xy$

$(e)f(x,y)=(xy{)}^3$

$($b$)f(x,y)=y-x$

$(f)f(x,y)=(xy{)}^3$

$($c$)f(x,y)=y-x^2$

$(g)f(x,y)=(x^2{y}^2{)}^3$

$($d$)f(x\frac{,}{b}ar(y))=1-x^2-y^2$

$($h$)f(x,y)=(x-y^2{)}^3$