21. Two seven-digit numbers $74 A 52 B 1$ and $326 A B 4 C$ are both divisible by 3. Then the value of $C$ is ( ).
(A) 1
(B) 2
(C) 3
(D) 5
(E) 8
21. A.

From the problem, we know
$$
\begin{array}{l}
7+4+A+5+2+B+1 \\
=19+A+B \equiv 0(\bmod 3) \\
\Rightarrow A+B \equiv 2(\bmod 3) \\
3+2+6+A+B+4+C \\
=15+A+B+C \equiv 0(\bmod 3) \\
\Rightarrow C \equiv 1(\bmod 3)
\end{array}
$$