6. Write down natural numbers starting from 1, all the way to the 198th digit:
$$
\underbrace{123456789101112 \cdots \cdots}_{198 \text { digits. }}
$$

Then the remainder of this number when divided by 9 is
(A) 4 ;
(B) 6 ;
(C.) 7 ;
(D) None of the above.
Answer $(\quad)$
6. (B)

Solution This number is
$1234 \cdots 9101112 \cdots 99100101102$.
Since $1+2+3+\cdots+9$ is divisible by 9, and $1+0+1+1+1+2+$ $\cdots+9+9$ (the sum of the digits of all integers from 10 to 99) is also divisible by 9, the remainder when this number is divided by 9 is
$$
1+0+0+1+0+1+1+0+2=6 .
$$