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# The locus represented by $xy+yz=0$ is

$\begin{array}{1 1}(A)\;A\; pair\; of\; perpendicular\; lines & (B)\;A\; pair\; of\; parallel\; lines\\(C)\;A\; pair\; of\; perpendicular\; planes & (D)\;A\; pair\; of\; parallel\; lines\end{array}$

Toolbox:
• $xy$ plane passes through the orgin and is perpendicular to $z-axis$
• $yz$ plane passes through the orgin and is perpendicular to $x-axis$
• $xz$ plane passes through the orgin and is perpendicular to $y-axis$
The given equation is $xy+yz=0$
Clearly $xy$ represents the plane passing through the orgin and perpendicular to $z-axis$
Clearly $yz$ represents the plane passing through the orgin and perpendicular to $x-axis$
Hence $xy+yz=0$ represents the locus a pair of perpendicular planes.
The correct option is $D$