\begin{tikzpicture}[->,>=stealth,shorten >=1pt,auto,scale=\rthinning,transform shape]
	% x node set with absolute coordinates
	\node[state] (A) at (1.5,2) {$A$};
	\node[dstate] (X) at (1.5,0) {$X$};
	\node[state] (C1) at (0,-1.7) {$C_1$};
	\node[state] (C2) at (3,-1.7) {$C_2$};
	\node[state] (D) at (1.5,-3.4) {$D$};
	% Directed edge
	\path (A) edge (X);
	\path (X) edge (C1);
	\path (X) edge (C2);
	\path (C1) edge (D);
	\path (C2) edge (D);
	% Bidirected edge
	%\path[bidirected] (A) edge[bend left=60] (C);
\end{tikzpicture}
