Take a four-digit number and multiply it by 3456, use A to represent the sum of the digits of the product, use B to represent the sum of the digits of A, and C to represent the sum of the digits of B, then what is C?
任取一个四位数乘3456，用A表示其积的各位数字之和，用B表示A的各位数字之和，C表示B的各位数字之和，那么C是多少．
<result>9</result>