(1, 4) node[circle, black, draw](a){A}
(2.5, 4) node[circle, black, draw](b){B}
(4, 4) node[circle, black, draw](c){C}
(1, 2.5) node[circle, black, draw](d){D}
(2.5, 2.5) node[circle, black, draw](e){E}
(4, 2.5) node[circle, black, draw](f){F}
(1, 1) node[circle, black, draw](g){G}
(2.5, 1) node[circle, black, draw](h){H}
(4, 1) node[circle, black, draw](i){I};