Make correct statements by filling in the symbols $\subset$ or $\not\subset$ in the blank spaces:
${ x:x \text{ is a circle in the plane} } \dots { x:x \text{ is a circle in the same plane with radius } 1 \text{ unit} }$

(B) The set on the left contains all circles in the plane,which includes circles of all possible radii (e.g.,$r=1, r=2, r=5$,etc.).
The set on the right contains only those circles in the same plane that have a radius of exactly $1$ unit.
Since there are many circles in the plane with radii other than $1$,the set of all circles is not a subset of the set of circles with radius $1$.
Therefore,the correct symbol is $\not\subset$.