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

with a pair $of$ equations.

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