Mario Lamas Espinoza

I have this definition
La clouse of a weigthed digraph $D$ is the vertex set $C$ of $D$ such that if $x \in C$ and $(x,y) \in$ arcs of D then $y \in C$

I don't understand this statement: "Consider a weighted digraph $D$. Let $D’$ be a spanning subdigraph of $D$ (i.e. $D’$
is obtained from D by removing some of the arcs). Let $C$ and $C’$ be maximum
closures of D and D’, respectively. Then
$ w(C) \leq w(C') $."