Hi, I need to align each row of the graph to the center. I am trying to do it with xshift. Here the code:

    \begin{tikzpicture}[node distance=1.5cm, auto, text centered]
    \tikzstyle{every node}=[draw,ball,align=center];
    \begin{scope}[xshift=1.5cm]
        \node (A) {A};
        \node [right of=A] (B) {B};
        \node [right of=B] (C) {C};
        \node [right of=C] (D) {D};
    \end{scope}
    \begin{scope}[yshift=-1.5cm]
        \node (AB) {AB};
        \node [right of=AB] (AC) {AC};
        \node [right of=AC] (AD) {AD};
        \node [right of=AD] (BC) {BC};
        \node [right of=BC] (BD) {BD};
        \node [right of=BD] (CD) {CD};
    \end{scope}
    \begin{scope}[yshift=-3cm,node distance=2cm,xshift=1cm]
        \node (ABC) {ABC};
        \node [right of=ABC] (ABD) {ABD};
        \node [right of=ABD] (ACD) {ACD};
        \node [right of=ACD] (BCD) {BCD};
    \end{scope}
    \begin{scope}[xshift=4cm, yshift=-4.5cm, node distance=2cm]
        \node (ABCD) {ABCD};
    \end{scope}
\end{tikzpicture}

Is there any other way to do it? Do not like to change xshift values every time.