A.width:0;height:0;border:10pxsolidtransparent;border-top-color:#00f;
B.width:10px;height:10px;border:10pxsolid#00f;border-top-color:transparent;
C.width:0;height:0;border:10pxsolidtransparent;border-bottom-color:#00f;
D.width:10px;height:10px;border:0pxsolid#00f;border-bottom-width:10px;