Bananabean:
In the set of numbers from 1 to 38, there are exactly two with property C (fourth power) - 1 and 16. So the grid entries at 1a and 16d have property C; they are both 4th powers. There are six numbers (1, 4, 9, 16, 25 and 36) with property A but 1 & 16 are already associated with C, so the four that are associated with A are 4, 9, 25, and 36, So 4d, 9d, 25d and 36d are all associated with A; i.e the entries are all squares.
It gets a lot harder after this...