The number of six digits is () A. 40B. 60C. 80D. 10
If the number of single digits is even, if 2 is in the single digit, then 1 is in the ten digit, and there are a22a22 = 4. If 2 is not in the single digit, there are A12 · A12 · A22 · A22 = 16. If the number of single digits is even, there are 4 + 16 = 20 six digits. Similarly, if the number of single digits is odd, there are 20 six digits that meet the conditions, and the number of such six digits is 40