Trying not to give too much of the theme away, the seven 21As are:

1A3, 3D2, 10A5
3D2, 10A5, 4D4
9D1, 11A2, 6D4
21A10, 23D2, 25A3
20D4, 28A3, 16D8
28A5, 16D8, 30A3
29A2, 26D4, 31A5