$In$ Exercises $25-34,$ describe the given set with a single equation $or$

with a pair $of$ equations.

$x,y,z$

$0,11,11$

The plane through the point $(3,-1,1)$ parallel $to$ the

$a.xy-$plane

$b.yz-$plane

$c.xz-$plane

The circle $of$ radius $2$ centered $at(0,0,0)$ and lying $in$ the

$a.xy-$plane

$b.yz-$plane

$c.xz-$plane

The circle $in$ which the plane through the point $(1,1,3)$ perpendic$-$

ular $to$ the $z-$axis meets the sphere $of$ radius $5$ centered $at$ the origin

The set $of$ points $in$ space that lie $2$ units from the point $(0,0,1)$

and, $at$ the same time, $2$ units from the point $(0,0,-1)$