Write a 4-digit number arbitrarily, such as 1996, multiply this number by 3456, and the product is A=1996*3456=6898176. Then add the digits of A, record as B=6+8+9+8+1+7+6=45, and then add the digits of B, record as C=4+5=9. If there are several friends, each write a 4-digit number and calculate A, B, and C respectively. After the calculation, as long as the calculation is correct, answer it. No matter what the 4-digit number was originally written, the final answer must be C= 9. why?
任意写一个4位数，如1996，把这个数乘3456，积记为A=1996*3456=6898176．然后把A的各位数字相加，记为B=6+8+9+8+1+7+6=45，再把B的各位数字相加，记为C=4+5=9．如果有好几位朋友，各写一个4位数，分别算A、B、C，算完以后，只要计算正确，对答一下，不管当初写的4位数是什么，最后答数一定是C=9．为什么呢？
<result>9</result>