digraph blockchain {
    rankdir=LR; // Flow from left to right
    node [shape=box, width=0.3, height=0.3];
    
    A [label=""];
    B [label=""];
    C [label=""];
    C1 [label=""];
    D [label="?", style=dashed];
    D1 [label="?", style=dashed];

    A -> B;
    B -> C;
    B -> C1;
    C -> D [style=dashed];
    C1 -> D1 [style=dashed];

    {
        rank=same;
        A;
    }
    {
        rank=same;
        B;
    }
    {
        rank=same;
        C;
        C1;
    }
    {
        rank=same;
        D;
        D1;
    }
}