6 In the following sum, $O$ represent the digit $0 . A, B, X$ and $Y$ each represents distinct digit. How many possible digits can $A$ be?
$$
\begin{array}{r}
A O O B A O O B \\
+\quad B O O A B O O A \\
\hline X X O X Y X O X X
\end{array}
$$
(A) 6
(B) 7
(C) 8
(D) 9
(E) 10
6. Answer: (A)
It is clear that $X=1$. So $A+B=11 \Rightarrow Y=2$. Hence $A+B$ can be $3+8$ or $4+7$ or $5+6$ or $6+5$ or $7+4$ or $8+3$.