6. (10 points) A number sequence $219 \cdots$, starting from the 4th digit, each digit is the unit digit of the sum of the previous 3 digits. Below are 4 four-digit numbers: $1113, 2226, 2125, 2215$, among which ( ) do not appear in the number sequence.
A. 1
B. 2
C. 3
D. 4
【Analysis】According to the problem, the sum of the digits of 219 is $2+1+9=12$, so the next number is the unit digit of the result, which is 2, making it 2192. Then, we can find the pattern of the numbers using the enumeration method.

【Solution】Solution: Enumeration method
The sum of the digits of 219 is 12, the next number is 2
The sum of the digits of 192 is 12, the next number is 2
The sum of the digits of 922 is 13, the next number is 3
The sum of the digits of 223 is 7, the next number is 7
The sum of the digits of 237 is 12, the next number is 2
Thus, the sequence of numbers is: $2192237221584790651281102 \cdots$
The pattern shows that the unit digit of the sum of the digits follows a cycle of two odd numbers followed by two even numbers.
Therefore, the answer is: $C$.