2. (10 points) Letters $A, B, C$ represent different digits from 1 to 9. In all cases where the following addition problem is correct, the product of the digits that the three letters $A, B, C$ cannot possibly take is $\qquad$

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将上面的文本翻译成英文，请保留源文本的换行和格式，直接输出翻译结果。
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The above text translated to English, with the original line breaks and format preserved, is as follows:

2. (10 points) Letters $A, B, C$ represent different digits from 1 to 9. In all cases where the following addition problem is correct, the product of the digits that the three letters $A, B, C$ cannot possibly take is $\qquad$

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Please translate the above text into English, preserving the original line breaks and format, and output the translation directly.
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【Analysis】First, analyze that $A$ plus the carry equals $B$, so $A$ is 1 less than $B$, and the sum of $A$ and $B$ involves a carry. We can enumerate all cases and exclude them accordingly.

【Solution】Solution: According to the problem, we know:
$A$ plus the carry equals $B$, so these two numbers differ by 1, which can be:
$$
\begin{array}{l}
A=5, B=6, C=1 \text {. } \\
A=6, B=7, C=3 \text {. } \\
A=7, B=8, C=5 \text {. } \\
A=8, B=9, \quad C=7 \text {. } \\
\end{array}
$$

Therefore, the numbers that $A, B, C$ cannot take are 2, 4, i.e., $2 \times 4=8$. Hence, the answer is: 8