def find_anchor_s(min_id_ind,
                  selected_x,
                  selected_y,
                  selected_z):
    """
    Finds the anchor S using the minimum ID of selected anchors.
    S anchor must be the anchor which has the minimum ID in selected
    anchors.
    """
    s_x = selected_x[min_id_ind]
    s_y = selected_y[min_id_ind]
    s_z = selected_z[min_id_ind]
    anch_s = [s_x, s_y, s_z]

    return anch_s