\begin{tikzpicture}[->,>=stealth,shorten >=1pt,auto,scale=0.6,transform shape]
		% x node set with absolute coordinates
		\node[state] (A) at (1.5,2) {$A$};
		\node[dstate] (X1) at (0,0) {$X_1$};
		\node[dstate] (X2) at (3,0) {$X_2$};
		\node[dstate] (Y1) at (0,-2) {$Y_1$};
		\node[dstate] (Y2) at (3,-2) {$Y_2$};
		\node[state] (C1) at (0,-4) {$C_1$};
		\node[state] (C2) at (3,-4) {$C_2$};
		\node[state] (D) at (1.5,-2.3) {$D$};
		% Directed edge
		\path (A) edge (X1);
		\path (A) edge (X2);
		\path (X1) edge (Y1);
		\path (X2) edge (Y2);
		\path (Y1) edge (C1);
		\path (Y2) edge (C2);
		\path (D) edge(C1);
		\path (D) edge (C2);
		% Bidirected edge
		%\path[bidirected] (A) edge[bend left=60] (C);
\end{tikzpicture}
