Suppose $G$ is a DAG with $n$ vertices, and $v$ is a vertex of $G$. What can we say regarding $v$ if the following holds:
A. In all topological sorts, $v$ is at the end of the list.
So my initial answer is that $v$ is the "sink" vertex, but in some case its not always correct, can't seem to find a feature that cant be said on $v$?
B. In all topological sorts, $v$ is at the top of the list.
So this one is the opposite which is the source vertex and has arrows coming out of him and no one points to him.