图表中的点和箭头

Question

请帮助我表示这个图表。

我不明白为什么箭头没有出现，而且我在表示点时也会遇到问题。

我的尝试：

    \begin{document}

\begin{center}
\begin{tikzpicture}
\begin{scope}[every node/.style={circle,thick,draw}]
\node[shape=circle,draw=black] (A) at (0,2) {$V_a$};
\node[shape=circle,draw=black] (B) at (2.5,2) {$V_b$};
\node[shape=circle,draw=black] (C) at (5,2) {$V_c$};
\node[shape=circle,draw=black] (D) at (7.5,2) {$V_d$};
\node[shape=circle,draw=black] (E0) at (-2,0) {$e_0$};
\node[shape=circle,draw=black] (E1) at (0,0) {$e_1$} ;
\node[shape=circle,draw=black] (E2) at (2,0) {$e_2$};
\node[shape=circle,draw=black] (E3) at (4,0) {$e_3$} ;
\node[shape=circle,draw=black] (E4) at (6,0) {$e_4$};
\node[shape=circle,draw=black] (E5) at (8,0) {$e_5$} ;
\node[shape=circle,draw=black] (E6) at (10,0) {$e_6$} ;
\end{scope}

\begin{scope}[every node/.style={circle,thick,draw}]
\path [->] (A) edge node[left] (B);

\end{scope}
\end{tikzpicture}
\end{center}


\end{document}

因此，请给我举一个指导性的例子（我没有发现任何类似的东西）。

谢谢你！