GC Focus P&C 1 Hard
A certain identification code consists of 6 digits, each of which is chosen from 1, 2, ..., 9. The same digit can appear more than once. If the sum of the 1st and 3rd digits is the 5th digit, and the sum of the 2nd and 4th digits is the 6th digit, how many identification codes are possible?
A. 625
B. 720
C. 1,296
D. 2,025
E. 6,561