20. Fill the letters ABC into the small squares in the figure, with the following requirements:
1) Each small square should either contain a letter or be left blank as an empty space;
2) Each row and each column must contain exactly one $\mathbf{A}$, one $\mathbf{B}$, and one $\mathbf{C}$;
3) Squares containing the same letter must not be adjacent (squares are considered adjacent if they share at least one vertex);
4) The letters and numbers outside the grid indicate the number of small squares between the corresponding letters in that row or column. After completing the grid, the sequence of English letters seen along the diagonal (following the arrow direction, with empty spaces represented by the letter $\mathbf{X}$) is?
【Answer】XXCXAC