================================= | A | B | C | D | ================================= | a1 | | | | |_______|_______|_______|_______| | a2 | | | | |_______|_______|_______|_______| | a3 | | | | |_______|_______|_______|_______| | a4 | | | | |_______|_______|_______|_______| | a5 | | | | =================================