Match the surfaces $z=f(x,y)$ with the verbal description $of$ the level curves

$(f(x,y)=k,k=1,2,3,$dots$)by$ placing the letter $of$ the verbal description $to$ the

left $of$ the number $of$ the surface.

$z=xy$

$z=x^2+y^2$

$z=\sqrt[\phantom{2}]{x^2+y^2}$

$z=\frac{1}{x-1}$

$z=2x^2+3y^2$

$z=2x+3y$