This is a periodic problem, so finding the period is the key.

The arrangement of typing letters is as follows: d b c c b d a a, the cycle seems to be too long, with 8 letters per cycle.

So the number 10 is B, and the number 20 12\8 is 4, so the number 20 12 is C.

If there is a better way, please tell me, thank you!