[B บ ....]

2. อันนี้งงโจทย์ครับ
Let $X$ be a topological space. Show that if $X$ is $T_1$, then the set of all limit points of each subsets of $X$ is closed.
คือเค้าให้แสดงว่า $A'$ is closed for all $A \subset X$ or $L=\{x\in X | x \in A' \ \mbox{for some subset}\ A \ \mbox{of} \ X.\}$ is closed.