Make correct statements by filling in the symbols $\subset$ or $\not\subset$ in the blank spaces:
${ x:x \text{ is a triangle in a plane} } \dots { x:x \text{ is a rectangle in the plane} }$

(B) Since a triangle is a polygon with $3$ sides and a rectangle is a polygon with $4$ sides,no triangle can be a rectangle.
Therefore,${ x:x \text{ is a triangle in a plane} } \not\subset { x:x \text{ is a rectangle in the plane} }$.